Session GP01 - Poster Session II.
POSTER session, Monday afternoon, March 22
Exhibit Hall, GWCC

[GP01.01] Collisional Properties of Ultracold Potassium: Consequences for Degenerate Bose and Fermi Gases

The hyperfine-state-selected scattering properties of potassium atoms at ultralow temperatures are calculated using accurate potentials gleaned from a recent analysis of photoassociation data. We predict that ^39K will possess a small, probably negative scattering length, which will hinder its evaporative cooling to the quantum degenerate regime, unless experiments take advantage of its magnetic-field-induced Feshbach resonance. The large, positive value calculated for the ^41K triplet scattering length makes it a better candidate for condensation at zero magnetic field. We also predict large, positive scattering lengths for experimentally relevant mixtures of spin states in the fermionic isotope ^40K. This result points to the likely possibility that ^40K may also be amenable to evaporative cooling. Moreover, certain spin states of ^40K also possess Feshbach resonances that may facilitate the creation of Cooper pairs in these gases. This work was supported in part by the National Science Foundation.

[GP01.02] Determination of ^39K Scattering Lengths Using Photoassociation Spectroscopy

Scattering lengths play an important role in the rapidly growing field of Bose-Einstein condensation in dilute atomic gases. To date, the scattering lengths are known quite accurately for Li, Na, Rb, and to a lesser extent Cs primarily through the analyisis of photoassociation experiments. We present the first theoretical analysis of the ^39K 0_g^- ro-vibrational spectrum which probes, via the free-bound Franck-Condon factors, the region of internuclear separation (R=40-80 a.u.) essential for determining the two-body scattering length a. Based on our fits of the spectral lineshapes and relative intensities we can restrict the bounds on the singlet and triplet scattering lengths to a_s = 138\pm 5 a.u. and a_t = -15\pm 30 a.u., respectively. In addition, the analyis incorporates the multichannel quantum defect ideas presented in Burke et. al, Phys. Rev. Lett. 81, 3355 (1998) dramatically increasing the effeciency of the calculations.

This work was supported in part by the National Science Foundation.

[GP01.03] Double-Resonance Photoassociative Spectroscopy of Ultracold ^39K Atoms near the Lowest Asymptote

We describe high resolution long-range molecular spectroscopy of ^39K_2 near the lowest 4^2S_1/2+4^2S_1/2 asymptote by \Lambda-type free-bound-bound double-resonance photoassociation of ultracold ^39K atoms. Hyperfine coupled bound levels within 5 GHz of the F''=1 + F''=1 threshold are observed in trap loss spectra via the pure long-range 0_g^- (v'=0, J'=2, R_c=52.2 a_0) intermediate state which dissociates to the 4^2S_1/2 + 4^2P_3/2 atomic limit. The binding energies of the near-threshold ground-state molecular levels allow accurate determination of the cold collision properties of potassium atoms, in particular the ground singlet and triplet scattering lengths. The triplet scattering length appears to be small and negative.

[GP01.04] What the Hamiltonian Structure of Ground State Alkali Atoms Tell us about K

An understanding of the Hamiltonian structure for two colliding ground state alkali atoms and the relationship between this structure and the singlet a_S and triplet a_T scattering lengths provided by nature is useful in interpreting experimental data based on cold collisions or photoassociation spectroscopy. As an example we will show how this information has been used in analyzing the experimental photoassociation spectra of the pure long-range ^39\rmK 1_u state in order to obtain a scattering lengths for K.

[GP01.05] Experimental study of ultra-cold metastable neon collisions

The odds for producing a Bose-Einstein condensate of metastable rare gas atoms depend to a large degree on (a) the sign and magnitude of the scattering length and (b) the rate of ionization in spin-polarized binary collisions at low temperatures (1). We have started an experimental program to try to determine these quantities for metastable neon by studying collisions in an atomic beam. Laser cooling is used to produce a sufficiently dense (>10^9/cm^3) and cold (<25mK) beam of metastable atoms for such studies (2). Ionization rates are determined with and without optical pumping into a spin-polarized state. Information about ground state potentials can be inferred from photoassociation spectroscopy. We hope to present our first experimental results at the conference. (1) M.R.\ Doery et al., Phys.\ Rev.\ A 58 (1998) 3673 (2) Details of the setup are shown on our web-site

[GP01.06] Observation of the Photoassociation Spectrum of Cold Cesium Atoms

Photoassociation spectra have been extensively studied in laser-cooled alkali atoms such as lithium, sodium, and rubidium. Analysis of these spectra yield important parameters such as the scattering length for cold collisions and precise determination of the lifetime of the upper atomic state for the relevant transition. Recent experiments have begun to study cesium atom photoassociation near the D2 (852 nm) transition(Fioretti, et. al.), Phys. Rev. Lett. 80, 4402 (1998).^,(Takekoshi, et. al.), to appear in Phys. Rev. A 59 (1999).. In this paper, we present photoassociation spectra obtained near the D1 (895 nm) transition in cesium. Laser-cooled and trapped Cs atoms are photoassociated using light from a tunable Ti:Sapphire laser near 895 nm. The Ti:Sapphire laser is mixed with light from a diode laser locked to the D1 atomic transition so that the absolute frequency of the light can be determined using a heterodyne technique. Further analysis of this data will increase the accuracy of the scattering length and atomic excited state lifetime for Cs.

[GP01.07] Ultracold atom-ion collisions: the case of Na+Na^+

Studies of charge exchange and total cross sections in elastic collisions of atom-ion alkali metals at ultralow temperatures are reported. Calculations for Na+Na^+ have been carried out with the best available ^2\Sigma_g and ^2\Sigma_u potential curves. As functions of energy, the cross sections show considerable structure and are large in the limit of low temperature. The scattering lengths were also computed, and the effective range expansion verified. For higher temperatures, we compare quantal and semi-classical results, and investigate the range of applicability of the Langevin formula. Even at temperatures of few degrees Kelvin, the charge exchange cross sections are large, and could provide an efficient way to produce cold ions.

[GP01.08] Analytical wavefunctions for ultracold H + H collisions

Exact wavefunctions describing molecular dissociation at large internuclear separations are utilized to calculate the scattering cross section of two hydrogen atoms at ultracold temperatures. The scattering calculations are performed by numerically integrating Schroedinger's equation over short distances, and matching the solutions onto analytical wavefunctions describing the relative motion of distant atoms. Accurate potentials are used in the short-range region, while the long-range solutions depend soley on well-known dispersion coefficients and the reduced mass of the two-atom system.

[GP01.09] High-precision calculations of van der Waals coefficients, polarizabilities, and atom-wall interaction coefficients for alkali-metal atoms.

The van der Waals interaction plays an important role in characterizing the ultra-cold temperature collisions between two ground state alkali-metal atoms and accurate knowledge of its value can impact predictions of whether a Bose-Einstein condensate is stable for a particular system. Other experiments are yielding constraints on magnitudes of the coefficients. We present results for C_6 dispersion coefficients, static dipole polarizabilities, and atom-wall interaction coefficients based on ab initio calculations of frequency-dependent dipole polarizabilities. We combine several many-body techniques to calculate various contributions to the polarizability. In particular, we used a linearized coupled-cluster method truncated at the level of single and double excitations to obtain the leading contribution from valence states. The contribution from autoionizing states which enters through the core polarizability is obtained within the framework of the relativistic random-phase approximation.

[GP01.10] Energy Transfer Collisions in a Sample of Cold Rydberg Atoms

In the last few years the idea of using trapped neutral atoms to produce cold Rydberg atoms has drawn special attention. With the advent of the MOT cold samples of Rydberg atoms, which are naturally Doppler-free, can now be produced. This achievement opens a complete new possibilities to study collisions involving Rydberg atoms. In this work we study the energy transfer collision process involving the 28P, 28S, and 29S excited states in cold Rubidium. The time evolution of the 29 S state population is followed by field ionization. Our setup is constituted by a standard Rubidium vapor cell MOT, using stabilized diode lasers for the trapping and repumping laser beams. A pulsed laser was used to excite the Rydberg atoms. The cold cloud of atoms is formed between two metal grids, separated by 2 cm from each other. In one of them we apply a fast HV pulse in order to ionize the Rydberg atoms. The ions are detected by a channeltron particle multiplier. By varying the delay between the light pulse and the HV pulse we can monitor the 29S population. A model based on rate equations, which takes in account radiative decays and energy transfer collisions, agrees well with the experiment results. Work supported by FAPESP and Programa Pronex.

[GP01.11] Quasiresonant energy transfer in ultracold atom-diatom collisions

We investigate ultracold He--H_2 collisions, and find that they are dominated by quasiresonant vibration-rotation energy transfer. Classical trajectory computations show that extremely strong correlations between \Delta j and \Delta v persist at low energies, leaving an adiabatic constant of the motion that is conserved to better than one part in 10^5. However, the classically allowed changes to j and v are substantially less than one quantum, so state-to-state transitions are classically forbidden. Quantum reaction rate computations show that quasiresonant transitions occur in the limit of zero collision energy, but that threshold effects become important and that some quasiresonant channels close. The qualitative similarity between classical and quantum results suggests that they share a common mechanism.

[GP01.12] The repulsive 1/r^3 interaction

Analytic solutions of the Schrödinger equation are presented for a repulsive 1/r^3 potential. They lead to an in-depth understanding of scattering by a pure repulsive 1/r^3 interaction including exact cross sections and phase shifts. Scattering by any potential which is asymptotically a repulsive 1/r^3 is also discussed. The results have applications in cold-atom collisions in a laser field and in many other quantum systems where the repulsive 1/r^3 interaction can be due to resonant electric dipole-dipole, magnetic dipole-dipole, or quadrupole-monopole interactions.

[GP01.13] Accuracy of Close-Coupling approaches using Single-Center Expansions for Positron-Atom Scattering

For positron-atom scattering, the break-up channel contains both the effects of ionization and positronium formation. In a single-center close-coupling expansion, these effects must be contained in the bound and continuum states. In previous R-matrix calculations for positron-lithium scattering, it was found that that the cross sections for exciting the 3d state were anomalously large and it has been argued that the atomic d-states try to represent the positronium formation channels and in so doing over estimate the cross sections for excitation to this state. It has further been argued that the only way to avoid this problem is to use a two center basis set expansion. We have investigated this hypothesis using a single-center convergent-close-coupling (CCC) method and have found that accurate N=2 and N=3 cross sections can be obtained using a single-center expansion even for energies just above the ionization threshold.

[GP01.14] Electron Attachment and Detachment: COT

Electron attachment to cyclooctatetraene (C_8H_8) has been studied over the temperature range 297-389 K using a flowing-afterglow Langmuir-probe apparatus. COT was chosen for study because recent experiments showed a large conformal change between the negative ion and neutral.^1 Photodetachment of the planar negative ion leads to a planar transition state for ring inversion in tub-like neutral COT. The question is, how does this unusual conformal change affect electron attachment? The rate constant for electron attachment (297-389 K) is 3.7 x 10^-9 cm^3/s (about 1 attachment in 100 collisions). The electron affinity of COT is low enough that thermal electron detachment takes place in our temperature range, and detachment rates have been measured; the activation energy for detachment is 190 meV. The electron affinity of COT may be deduced from the equilibrium constant (0.59 eV) and will be compared to the results of other work. 1. P.G. Wenthold, D.A. Hrovat, W.T. Borden, and W.C. Lineberger, Science 272, 1456-1459, 1996.

[GP01.15] Electron Excitation at Forward Scattering.^1

The Regge pole representation of scattering problems embodies deep physical insights [1] and is not a merely contrived mathematical convinience. No physical understanding of angular distributions of a molecular collision process is achieved through summing a partial wave series with significant terms [2]. At forward scattering, the recent generalized Lassettre expansion [3], the momentum dispersion method [4] and the forward scattering function [5] all yield results for H~1s--2p excitation that agree excellently with those obtained by the convergent close--coupling approximation [6] down to about twice threshold energy. Measured electron DCS_s for Cd and Xe are used to demonstrate the utility of the analytical continuation of the measured data in angle and energy. This should encourage experimenters to measure in the difficult to access angular regime near and at \theta =0^\circ. \vglue .1in ^1Supported by NSF and DOE Division of Chemical Siences, Office of Basic Energy Sciences, Office of Energy Research. \vglue .1in \begintabularll [1]& J. N. L. Connor, J.~Chem. Soc. Farad. Trans. 86, 1627(1990) [2] &D. Sokolovski et.~al., Chem. Phys. Lett. 238, 127(1995) [3] &Z. Felfli et.~al., Phys. Rev. Lett. 81, 963(1998) [4] &A. Haffad et.~al., Phys. Rev. Lett. 76, 2456(1996) [5] &N. Avdonina et.~al., J. Phys. B 30, 2591(1997) [6] &I. Bray et.~al., Phys. Rev. A 44, 5586(1991) \endtabular

[GP01.16] Measurement of the electron--impact excitation cross sections for n = 1 \rightarrow 2 transitions in heliumlike and hydrogenlike xenon

Electron--impact excitation cross sections are important parameters for modeling energy transport mechanisms and are crucial for developing accurate spectral diagnostics in high--temperature plasmas. At the Livermore high--energy electron--beam--ion--trap facility (SuperEBIT) the K\alpha transitions in heliumlike Xe^52+ and hydrogenlike Xe^53+ have been measured using a transmission--type crystal spectrometer. The spectral resolution is sufficient to resolve the fine structure of the xenon K\alpha spectrum and, thus, to perform level--specific measurements of the electron--impact excitation cross sections. In particular, the determination of the cross sections includes the 1s^2\, 1\! S_0 \rightarrow 1s2p^\, 1\! P_1 , 1s^2\, 1\! S_0 \rightarrow 1s2p^\, 3\! P_1 , and 1s^2\, 1\! S_0 \rightarrow 1s2s^\, 3\! S_1 transitions in heliumlike Xe^52+ and the 1s_1/2 \rightarrow 2p_3/2 , and 1s_1/2 \rightarrow 2p_1/2 transitions in hydrogenlike Xe^53+. The uncertainties of the measured cross sections are in the 10%\ range and are mainly due to low counting statistics. Comparison with theoretically predicted values shows that agreement between the measured and calculated values can only be found when the Breit interaction is included in the calculations.

[GP01.17] Electronic excitation of hydrocarbons: Methylene and Ethylene

Electron collisions with polyatomic systems within the R-matrix methodology (L. A. Morgan, J. Tennyson and C. J. Gillan Comp. Phys. Commun. 114) 120 (1998). (L. A. Morgan, and J. Tennyson Phil Trans. B in press) (1998). is currently being undertaken as part of a UK wide collaborative project based on the SWEDEN-MOLECULE Gaussian code. Data on electron collisions with various hydrocarbons are required for many practical purposes such as plasma chemical processing and deposition, gas laser modeling, astrophysical processes, atmospheric modeling and for fundamental spectroscopic purposes. The present work concentrates on electron collisional excitation for the methylene radical (CH_2) and the ethylene (C_2H_4) complex. A multi--state CI approximation is used for both polyatomic systems where the lowest three target states are included in the CI expansions using pseudo-natural orbitals. Our calculations allow comparisons to be made directly with existing ab initio work performed using the Schwinger and Kohn methods and with the available experimental data. Detailed cross section comparisons will be made and a comprehensive set of results will be presented at the meeting.

[GP01.18] Electron collisional excitation of doubly ionized Neon

Observations on the spectra of doubly ionized neon (NeIII) have been recently recorded below 25O Å(A. E. Livington, R. Buttner, A. S. Zacarias, B. Kraus, K-H Schartner, F. Folkmann and P. H. Mokler, J. Opt. Soc. Am. B 14) 522-525 (1997).. This work together with previous studies give line intensies which may be used as density diagnostics but required accurate collision strengths and transition probabilities for their interpretation. We are currently undertaking detailed calculations for electron collisions with NeIII ions performed using the R-matrix method within the Breit-Pauli approximation. Multi-state configuration interactions wave functions incorporating the lowest lying twenty eight LS-coupled target states of NeIII are used in our work. Calculations for cross sections and rates are in progress and a comparison will be made with previous work which included only four LS-coupled states of NeIII (K.Butler and C. Mendoza, Mon. Not. R. Astr. Soc. 208) 17-23 (1984).. Further details and results will be presented at the meeting.

[GP01.19] Effective collision strengths for fine-structure forbidden transitions among the 3s^23p^3 levels of Cl~III

Electron temperatures and densities are difficult to determine in many astrophysical plasmas. However, it is well known that diagnostics on forbidden line intensity ratios for ions in the phosphorous isoelectronic sequence are of great importance in astrophysics, particularly for nebulae. A key element in the analysis is highly accurate atomic data. In this work we extend the earlier calculations of Butler and Zeippen (Astron. Astrophys. 208 337 (1989)) on electron scattering by Cl~III. We have obtained effective collision strengths for a wide range of electron temperatures using the R-matrix method. Twenty-three LS target eigenstates are included in the expansion of the total wavefunction, consisting of the seven n=3 states with configuration 3s^23p^3 and 3s3p^4, twelve n=3 states with configuration 3s^23p^23d, and four n=4 states with configuration 3s^23p^24s. The fine-structure collision strengths have been obtained by transforming to a jj-coupling scheme using the JAJOM program of Saraph (Comp. Phys. Commun. 15 247 (1978)) and have been determined at a sufficiently fine energy mesh to delineate properly the resonance structure. Results for both collision strengths and for effective collision strengths will be presented at the conference and comparison will be made with the earlier work.

[GP01.20] Simultaneous Electron-Impact Ionization--Excitation of Quasi-Two-Electron Atoms.

The general R-matrix code for electron-impact ionization~[1] has been modified (i)~to allow for high incident electron energies within the Plane-Wave Born Approximation (PWBA) and (ii)~to improve upon the description of both the target ground state and the interaction of the ejected electron with the residual ion via an R-matrix with pseudo-states (RMPS) approach~[2]. Results for cross sections (total, \hboxsingle-, \hboxdouble-, and triple-differential) as well as parameters measured in electron--photon coincidence experiments~[3] for ionization resulting in excited ionic states will be presented and compared with experimental data and predictions from other theoretical approaches. \par\medskip 1. K.~Bartschat, Comp.~Phys.~Commun.~75, 219 (1993) 2. K.~Bartschat, Comp.~Phys.~Commun.~114, 168 (1998) 3. M.~Dogan et al., J.~Phys.~B~31, 1611 (1998)

[GP01.21] (e,2e) Studies of Helium Autoionizing Levels

The helium 2\ell2\ell' autoionizing region is being investigated using (e,2e) spectroscopy. The present experiments are similar in concept to recent (e,2e) experiments in cadmium.(N.L.S. Martin, D.B. Thompson, R.P. Bauman and M. Wilson, Phys.\ Rev.\ A 50), 3878 (1994). Pairs of (e,2e) energy spectra for a given incident energy and scattering angle are taken at ejected-electron directions 180^\circ apart. The sum and difference of such spectra give information about interference cross-terms between different multipole partial wave amplitudes. Experiments are being carried out for incident electron energies of up to 500~eV.

[GP01.22] Electron-Impact Excitation of Neon from both the ground level and metastable levels

Knowledge of electron-impact excitation cross sections from both the ground state and metastable levels of atoms is needed for both a comprehensive understanding of the electron excitation process and the modeling of plasmas and discharges. We have previously studied excitation into the ten levels of the 3p^54p configuration of argon from both the ground level,(J. E. Chilton et al.), Phys. Rev. A 57, 267 (1998). and metastable levels.(G.A. Piech et al.), Phys. Rev. Lett. 81, 309 (1998). We are now in the process of studying the analogous excitation cross sections in neon. In contrast to argon, the pressure effects in the ground state excitation cross sections are much less. The cross sections for excitation processes out of the 1s_5 (^3P_2) metastable level of neon corresponding to optically allowed transitions (into 2p^53p levels with J=1,2,3) are very large with a broad dependence on the incident electron energy- similar to what we have found for argon.

[GP01.23] Use of Trapped Rb Atoms as a Target for Measurements of Electron-Impact Cross Sections

Trapped atoms are a unique target for measuring cross sections for electron-impact ionization and excitation.(R. S. Schappe et al.), Phys. Rev. Lett. 76, 4328 (1996). A special property of atom traps, which we now wish to exploit, is that a substantial fraction of the atoms are pumped into the 5^2P_3/2 excited level by the trapping radiation. Thus it is possible to obtain cross sections out of both the 5^2S ground level and the 5^2P excited level by comparing the signals obtained with the trapping laser on (5^2S and 5^2P) and with the lasers momentarily turned off (5^2S alone). We are presently working on determining the ionization cross section out the Rb(5^2P_3/2) excited level by measuring the subsequent change in the trap loss rate. We are also developing a newer trapping apparatus to measure excitation cross sections by detecting the fluorescence given off by electron-impact excited atoms. Preliminary results for Rb(5^2S \rightarrow 5^2P) excitation will be presented.

[GP01.24] Measurement of electron excitation cross sections of the 3p^54p, 3p^53d, 3p^55s levels of argon using Fourier transform spectrometry

Advances in Fourier transform spectrometers (FTS) allow previously unexamined excited levels of argon to be studied through detection of their infrared (1 - 6 \mu m) emissions. We find that radiation from the 3p^54p levels excited by an electron beam exhibits nonlinear dependence on gas pressure. We attribute this to the reabsorption of resonant photons by higher levels coupled to both the ground and the 3p^54p levels. We measure infrared transitions from the dominant 3p^55s and 3p^53d cascading levels, which display similar pressure effects. Subtraction of these cascade processes from the observed 3p^54p \rightarrow 3p^54s signals yields the direct electron excitation cross sections for the 3p^54p levels, which remain independent of gas pressure. The prominent cascades into the 3p^55s and 3p^53d levels can also be studied with the FTS, and the direct electron excitation cross sections extracted. We present direct cross sections for all 3p^54p levels, and the J = 0, 2, 3, 4 3p^55s and 3p^53d levels. Apparent cross section data at various energies and gas pressures are also presented.

[GP01.25] Absolute partial cross sections for electron-impact ionization of CO from threshold to 1000 eV

Absolute partial cross sections for the production of both parent and fragment ions from electron-impact ionization of CO are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected using a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values for singly charged parent ions is \pm4% and is slightly greater for fragment ions. Previous cross section measurements are compared to the present results.

[GP01.26] Electron Impact Excitation to and from the Metastable States of Kr I

Electron impact excitation rates to and from the metastable levels of noble gases such as argon, krypton, and xenon are essential inputs to interpreting rare gas actinometry as a diagnostic of molecular dissociation in plasma processing applications. These rates are also important for spectral analysis of lighting discharges and modeling of excimer laser amplifiers. We have already calculated the excitation cross sections to and from the metastable levels of argon using a distorted-wave method and find very large cross sections from the metastable levels. These cross sections will thus significantly affect the level populations of the metastables for any diagnostics. We present similar calculations of excitation cross sections from the ground and metastable levels for neutral krypton using DW method and including polarization for incident electron energies up to 100 eV. Unlike the argon case, there exist relatively little experimental data or theoretical calculations for electron impact excitation of neutral krypton. We will compare and contrast our results with those available at present.

[GP01.27] Inelastic Scattering of Electrons from Singly-Ionized Oxygen

Cross sections for the electron impact excitation of the ground 2s^22p^3~^4S^o state to the 2s^22p^3~^2D^o, ^2P^o, and 2s2p^4~^4P states of singly-ionized oxygen are computed in a 28-state close coupling approximation using the R-matrix method. Extensive configuration interaction wave functions are used to represent twenty eight LS states of the 2s^22p^3, 2s2p^4, 2s^22p^23s, 2s^22p^23p, 2s^22p^23d, and 2s^22p^24s configurations of O II. These wave functions are also used to calculate excitation energies, oscillator strengths of electric-dipole-allowed transitions among various states, and radiative lifetimes of excited states. The calculated excitation energies are in close agreement with recent laboratory measurement. Our calculated oscillator strengths, lifetimes, and cross sections will be compared with other calculations and experiments.

[GP01.28] Cross Sections for the Scattering of Electrons by Atomic Nitrogen

R-matrix method is used to calculate the differential and integral cross sections of the forbidden 2s^22p^3~^4S^o - 2s^22p^3~^2D^o transition for the scattering of electrons from atomic nitrogen in a 11-state close-coupling approximation. Extensive configuration interaction wave functions are used to represent target states which give excitation energies in close agreement with experiment. The calculated differential cross sections are peaked in the backward direction and compare very well with the recent measurement. The integral cross sections show very good agreement with the available other calculations and measurement.

[GP01.29] Heteronuclear hydrogen molecular ion: revisited

We revisit a fundamental and familiar problem in molecular spectroscopy, HD^+. Unlike its brethren, H^+_2 and D_2^+ whose Born-Oppenheimer (BO) Hamiltonian separates in prolate-spheroidal coordinates thereby preserving the symmetry under exchange of the nuclei, the physics of HD^+ is profoundly different. Owing to the mass difference between the proton and deuteron, the symmetry under the exchange of the nuclei is broken, and the adiabatic potential energy curves now nearly cross within each molecular symmetry and give rise to non-adiabatic transitions. A far more profound influence of this mass difference is that because the geometric center and the nuclear center of mass do not coincide, a permanent electric dipole is formed in the ground and excited electronic states. Historical developments in theoretical treatment of HD^+ have begun with a body-fixed coordinate system with the origin placed at the geometric center of the nuclei. H_2^+ electronic wavefunctions are used at a later stage to obtain the eigenvalues of the symmetry-breaking operator. We begin with a treatment of the BO Hamiltonian in the center of mass of the nuclei and rewrite the Hamiltonian in prolate-spheroidal coordinates with the origin at the center of mass. In this fashion, we explicitly include information about the reduced nuclear mass in the BO adiabatic Hamiltonian. The adiabatic potential energy curves which we obtain give the correct dissociation thresholds. We present precise calculations of the HD^+ potential energy curves, coupling matrix elements, transition dipole matrix elements, varational energies and transition frequencies. We are motivated by a need for spectroscopic accuracy. We are also motivated to investigate effects beyond the adiabatic approximation for which we can calculate accurate coupling matrix elements.

[GP01.30] A Full Quantum-Mechanical Calculation Involving Low-Rydberg Alkali Atoms

To assess the reliability and limitation of the semi-classical Molecular Orbital approximation for estimating the depopulation cross sections of low-Rydberg alkali atoms in the thermal energy region, a 2-channel full quantum-mechanical calculation on these atoms colliding with the ground state He has been reported. A comparative study between the results reveals that the semi-classical approach is reliable and accurate for v > 5 x 10^-4 a. u.. Extreme low-energy results are found sensitive to various quantal effects.

[GP01.31] Status of 2s^2p-2s2p^2-2p^3 transition probabilities in B-like ions: theory and experiment

Transition probabilities are calculated for 49 electric dipole 2s^22p--2s2p^2--2p^3 transitions in boronlike ions with nuclear charges ranging from Z=5 to 100. Relativistic many-body perturbation theory (MBPT), including the Breit interaction, is used to evaluate retarded E1 matrix elements in length and velocity forms. The calculations are carried out with two potentials: a non-local 1s^2 Dirac-Fock potential and a local 1s^2 model potential. We use first-order perturbation theory to obtain intermediate coupling coefficients, and the second-order MBPT to determine the matrix elements. The contributions from negative-energy states are included in the second order E1 matrix elements to achieve precise agreement between length-form and velocity-form amplitudes. The transition energies used in the calculation of oscillator strengths and transition rates are obtained from second-order MBPT. Transition probabilities are compared with critically evaluated experimental values and with results from other recent calculations.

[GP01.32] Relativistic many-body calculations of energy levels, hyperfine constants, and dipole matrix elements for alkali metal atoms.

We calculate removal energies and hyperfine constants for ns, np_1/2 and np_3/2 states of Na (n=3,4,5,6,7), K (n=4,5,6,7), Rb (n=5,6,7,8) and Cs (n=6,7,8,9). We also calculate removal energies for the ground state and several excited states of Fr. The calculations are carried out using relativistic all-order method including single, double and partial triple excitations of the Hartree-Fock ground state (SDpT). We use the SDpT wave functions to evaluate electric-dipole (E1) reduced matrix elements for transition between these states. We found that single-double approximation gives accurate results for removal energies and dipole matrix elements for the principal transition. The hyperfine constants for ^23Na, ^39K, ^85Rb, ^133Cs are found to be in good agreement with experiment after triple excitations are partially included. These calculations provide a basis for evaluation of PNC amplitudes in Cs and Fr.

[GP01.33] Strong Electric-field Effects on the Structure Profiles of Doubly Excited Resonances in He Ground State Photoionization

Electric-field effects on the doubly excited resonance structures in He ground state photoionization are investigated theoretically using the complex-rotation method(Y.K. Ho, Phys. Rep. 99), 1 (1983); A. Buchleitner, B. Gremaud, and D. Delande, J. Phys. B 27, 2663 (1994). with B-spline-based basis(T.N. Chang, in Many-body Theory of Atomic Structure and Photoionization), edited by T.N. Chang (World Scientific, Singapore, 1993), p.213.. States up to L_max=3 are coupled together by the external electric field for both initial and final states. The variations of the resonance structure profiles and Fano q-parameters for the M=0 components of He (2,na) (n=2-5) ^1P^o and ^1D^e doubly excited resonances series for selected DC electric field strengths are examined. The change of the resonance energies and widths will also be presented.

[GP01.34] Determination of Resonance Energies and Widths of Mg Doubly Excited States above the 3s and/or 3p Thresholds by the Stabilization Method with the B-spline-based Configuration Interaction Approach

The resonance energies and widths of Mg ^1P^o, ^1D^e, and ^1F^o doubly excited states above the 3s and/or 3p thresholds are determined by calculating the density of resonance states using the stabilization method(V.A. Mandelshtam, T.R. Ravuri, and H.S. Taylor, Phys. Rev. Lett. 70), 1932 (1993). with the B-spline-based configuration interaction (BSCI) approach(T.N. Chang, in Many-body Theory of Atomic Structure and Photoionization), edited by T.N. Chang (World Scientific, Singapore, 1993), p.213.. The effects due to the intrashell core excitation and intershell core-valence interactions are taken into account in the BSCI calculations by using a parametrized long-range core-polarization potential. The procedure of extracting the resonance energies and widths from the stabilization diagram, and comparisons with other results in the literature will be presented.

[GP01.35] Electron Affinity of He^- 1s2s\,^3S

In Kristensen, et al.(P. Kristensen, et al., Phys. Rev. A 55), 978 (1997)., the electron affinity of He 1s2s\,^3S has been determined to high precision from both theory and experiment. The predicted affinity is 77.518(11) meV. Most of the uncertainties in this result come from the calculated nonrelativistic energy, --2.178\,077\,85(32) a.u. To further reduce this uncertainty, a large scale computation is carried out with a correlated wave function. Our 4258 term wave function gives an energy upper bound of --2.178\,078\,033 a.u. for He^- 1s2s2p\,^4P^o. The extrapolated energy is --2.178\,078\,044(11) a.u. Using this energy, the electron affinity becomes 77.5236(30) meV. The theoretical uncertainty is reduced by a factor of four. The predicted affinity agrees with the experimental result of 77.516(6) meV^2 .

[GP01.36] Operator expansion for relativistic multiconfiguration calculations

Relativistic multiconfiguration calculations( F.A. Parpia, C. Froese Fischer and I.P. Grant, C.P.C. \textbf94,) 249 (1996) may involve large numbers of N-electron basis states formed from a few reference states by single and double substitutions to virtual orbitals. We write a matrix element of the hamiltonian between such basis states as a sum of 2-electron matrix elements multiplied by angular coefficients. We have succeeded in separating these angular coefficients into two kinds of quantities: (1) analytic factors which are explicit expressions involving the quantum numbers (n,l,j) of the virtual orbitals, and (2) numerical factors, independent of the virtual orbitals, containing the complications of antisymmetrization and coupling schemes. This allows the size of a calculation to be increased without recomputing the angular coefficients, and without increasing the amount of stored angular data. We present the analytic results and an outline of the derivations, done with the help of diagrammatic methods.(A.P. Jucys and A.A. Bandzaitis, \textitThe Theory of Angular Momentum in Quantum Mechanics) (Mokslas, Vilnius, 1977) Some numerical examples are also given.

[GP01.37] Alloying Behavior of some FCC Transition Metals from Simulation

The many-body, long-range potentials developed by Sutton-Chen [A. P. Sutton, J. Chen, Phil. Mag. Lett. \textbf61, 139 (1990)] (SC) are tested in the atomistic simulations of binary f.c.c. metallic alloys. As an example the alloying behavior of Ag-Au and Cu-Ni are studied at 300K using the Molecular Dynamics (MD) method. The MD algorithms that we use are based on the extended Hamiltonian formalism and the ordinary experimental conditions are simulated using the constant-pressure, constant-temperature (NPT) MD method. The enthalpy of mixing and density values of the random Ag-Au and Ni-Cu binary alloys are obtained as a function of concentration after 20000 to 25000 steps of NPT MD simulations. Simulation results are compared with the statically calculated values of Rafii-Tabar and Sutton [H. Rafii-Tabar and A. P. Sutton, Phil. Mag. Lett. \textbf63, 217 (1991)] and experiment.

[GP01.38] Theoretical Energy Levels of Fr-Like Ions.

The Dirac-Fock (DF) method has little difficulty in reproducing the energy level ordering and spacing of alkali atoms, Li through Cs, with a moderate accuracy even with the simplest wave functions, i.e., single configuration (SC) DF wave functions. However, SCDF wave functions for Fr-like ions (Z> 87) cannot even reproduce the level ordering of the valence electrons: 7s, 7p, 6d, and 5f including their fine structures. The exact positions of the 6d and 5f levels of Fr (Z=87) are not known, though the 5f_5/2 and 5f_7/5 levels are expected to be high. The 5f levels come down along the isoelectronic sequence of Fr, and eventually the theoretical level ordering becomes hydrogenic at Pa^4+ (Z=91), i.e., 5f_5/2 becomes the ground level followed by 5f_7/2, 6d, 7s, and 7p in that order. However, experimental level ordering for Th^3+ (Z=90) is already hydrogenic.(J. Blaise and J.-F. Wyart, Energy Levels and Atomic Spectra of Actinides), Tables Internationales de Constantes, Paris (1992). Reproducing the experimental positions of the 5f_j levels for Fr-like ions seems to be an extremely difficult theoretical challenge because both relativity and electron correlation must be treated fully and equally. We will report on the progress of alternative theoretical approaches we are testing to challenge this seemingly simple, yet demanding task.

[GP01.39] Hyperfine Structure Coupling Constants for He-like Ions

High precision calculations of the hyperfine structure coupling constants for the 2S and 2P states of He-like ions up to nuclear charge Z = 10 will be presented. The calculations include electric quadrupole interactions, and estimates of the relativistic, quantum electrodynamic, and finite nuclear size corrections. Results have been obtained for all known isotopes for which nuclear moments are available. In other cases, the calculations can be used to deduce the nuclear moments from measured hyperfine splittings. A comparison with experimental hyperfine splittings shows good agreement over the entire range of nuclear charge 2 \le Z \le 10 within the accuracy of the data, although there is some indication of a discrepancy which grows in proportion to Z^4.

[GP01.40] Simulation of Atomic Clocks with Unitary Integration

Unitary Integration(B. A. Shadwick and W. F. Buell, Phys. Rev. Lett., 79), 5189 (1997). is a numerical method that preserves the structure of the Liouville-von Neumann equation by evolving the density matrix \rho via unitary transformations, thereby preserving the kinematic invariants c_j=tr\rho^j,\ j=1,\ldots,n to all orders in the time step. Unitary integrators have been shown to be of particular utility when the time scales of interest are much greater than the characteristic time scales of the interaction. Atomic clocks represent such a system since the atomic coherence undergoes many oscillations in the course of a Ramsey interrogation time. In addition, high-Q atomic clocks represent a system where dissipation is weak. Unitary integration combined with an operator splitting approach to non-unitary evolution is an ideal technique for modeling such systems. We present results of numerical simulation of an atomic clock based on Raman resonance in a \Lambda system.

The Aerospace Corporation The Institute for Advanced Physics

[GP01.41] Density-Matrix Descriptions of Electromagnetic Interactions in Quantized Electronic Systems

Density-matrix descriptions have been developed to investigate the influence of relaxation phenomena during resonant and non-resonant radiative transitions of quantized electronic systems, including atomic systems and quantum-confinement systems (e. g., semiconductor microstructures). Radiative and collisional relaxation phenomena have been treated using Liouville-space projection-operator techniques. Both time-independent (resolvent-operator) and time-dependent (equation-of-motion) formulations have been developed. The self-energy operators that occur in these formulations can provide the basis for a self-consistent determination of the non-equilibrium and coherent electronic-state kinetics together with the spectral-line shapes.

[GP01.42] Calculation of Regge Trajectories Using their Analytic Properties in Energy.

Regge trajectories, generated by a superposition of Yukawa potentials, are evaluated using a simple (avoiding any kind of regulator) and efficient method \footnote C. Lovelace and D. Masson, Nuovo Cimento, Vol. XXVI, 3 (1962). based on the high energy Taylor series expansion of the trajectories. Their analytic properties in energy have been investigated in depth using Padé Approximations to effect the analytic continuation. Our study shows that different Regge trajectories are analytic continuation of one another in different Rieman sheets. Regge pole representation of scattering problems embodies deep physical insight; it leads to a new physical interpretation of diffraction scattering ( J. N. L. Connor, J. Chem. Soc. \small FARADAY TRANS.) 86, 1627 (1990).. Recently, it has been applied successfully to electron scattering \footnote Z. Felfli et al., Phys. Rev. Lett. 81, 963 (1998).. Our results are compared with those of previous authors \footnote D. Sokolovsky, C. Tully and J. Crothers, J. Phys. A31 1 (1998)..

Supported by NSF and DoE, Div. of Chem. Sciences, OBES, OER.

[GP01.43] Representability of reduced density matrices for many-electron systems

The ground state variational problem for an N-electron system may be formulated as one of linear optimization over the convex set of N-representable two-body reduced density matrices (RDM's) (A.~J.~Coleman, Rev.\ Mod.\ Phys.\ 35) (1963) 668--689; Claude Garrod and Jerome K.~Percus, J.\ Math.\ Phys.\ 5 (1964) 1756--1776.. We have found a new family of representability conditions by considering the expression I(k) = Tr\,\Gamma'.\Gamma-k\,Tr\,\gamma'.\gamma+k(k+1)/2, where (\gamma,\Gamma) are the one-body and two-body RDM's of the N-body system, (\gamma',\Gamma') are RDM's of an N'-body test system, and k is an integer in the range 0\leq k<\min(N',N). The known P and Q conditions are associated with N'=2; among our new conditions is I(1)\geq0 for N'=3. However, a conjecture that always I(k)\geq0 fails at \min(N,N')=4. In related work (Shidong Jiang et al., this Centennial Meeting.) we are re-investigating the practical application of semidefinite programming to the calculation of ground state properties, and are studying numerically the strength of old and new representability conditions.

[GP01.44] On Magnetic Dipole Interactions

We formulate a Dirac Hamiltonian for an electron in the field of a charged magnetic dipole, which can be solved exactly in the central field approximation. While no scattering resonances are found, we do find and describe three categories of bound-state solutions for this Hamiltonian which may represent the electron/positron system or possible new electromagnetic composites. One category of solution is a state with very strong binding energy (greater than the electron rest mass) but with only a small amount of kinetic energy. This state corresponds to a classical orbit picture in which the electric and magnetic forces are opposed and nearly balance each other. No bound states are found if the central magnetic dipole is uncharged.

[GP01.45] Using a Single Cycle Propagator to do Floquet Theory

We present examples of the use of a single cycle propagator in two-level and multi-level systems undergoing a sinusoidal interaction. This is equivalent to doing Floquet theory except that all multiphoton orders are included automatically. Among other things, this method is useful for strong oscillating fields with envelopes that are slowly varying compared to the frequency. The details of treating multi-level systems are discussed and the results are compared with experimentally measured values of the multiphoton Rabi frequency in the potassium 21s-19f system.

[GP01.46] Imaging Strong-Field Atomic and Molecular Dynamics

Imaging photoions and photoelectrons subsequent to strong-field ionization (atoms) and dissociative-ionization (molecules) is proving to be a valuable tool in deciphering complicated ejection dynamics. Recently, we have exploited our ability to collect ions and electrons over 4\pi sr to study angular and energy correlation between simultaneously ejected ions and electrons in atomic and molecular systems. In this poster we will present our results of electron-proton angular correlation subsequent to multiphoton dissociative-ionization of H_2 and triple correlation between atomic ions resulting from the total breakup of linear and bent triatomic systems. In addition, we will update our progress in the direct detection of non-sequential double ionization in atoms using our imaging spectrometer.

[GP01.47] Agreement of quantum and semiclassical approaches for above threshold ionization (ATI) of atomic Hydrogen in Rydberg states

Transition rates for two-photon ionization of H when photoionization is possible (ATI) don't have resonances. Therefore two-photon ATI of H in Rydberg states is convenient to investigate agreement between perturbation and semiclassical theories. To calculate ATI of H in the dipole approximation Sturmian expansion of Coulomb Green's function (CGF) is widely used. That allows to calculate ionization rates in states with main quantum number n<10. We got a modified Sturmian expansion for CGF that consists of two parts. The first part is similar to the well known expression of CGF with Whitteker's functions except that radial variables must not be interchanged. The other part is an infinite sum. Using modified CGF two-photon ionization rates for H in nl states are calculated for l up to 36 and n in some cases even up to 120. Ionization rate as function of n and \lambda/n^2 is presented in form of 2D and 3D graphics. Ionization rate Q_nl/I values for n\geq30 (l=1,2...10) are very close to each other. Semiclassical expressions for Q_nl and Q_n got by Bersons were used to compare quantum and quasiclassical theory predictions. There are good agreement for Q_n/I when n>10. At n>20 and 0

[GP01.48] On the Generation of Circularly Polarized Multiple High-Order Harmonic Emission From Two-Color Crossed Laser Beams

We present a scheme for the production of circularly polarized multiple high-order harmonic generation (HHG)( Xiao-Min Tong and Shih-I Chu, Phys. Rev. A 58) (1998) 2656.. The proposed experimental setup involves the use of two-color laser fields, consisting of a circularly polarized fundamental laser field and a linearly polarized second-harmonic laser field, in crossed beam configuration. The feasibility of such a scheme is demonstrated by an ab initio\ quantal study of the HHG power spectrum of He atoms by means of the time-dependent density functional theory with optimized effective potential and self-interaction correction recently developed( Xiao-Min Tong and Shih-I Chu, Phys. Rev A 57) (1998) 452.. The theoretical study also provides new insights regarding the different mechanisms responsible for the production of HHG in different energy regimes as well as the mechanism for the generation of continuous background radiation.

[GP01.49] Wavelet Time-Frequency Analysis of the Mechanisms in Multiple High-Order Harmonic Generation in Intense Pulsed Laser Fields

The quasiclassical three-step approximation (tunneling, free electron propagation in the laser field, and recombination with atomic core to emit radiation) is a popular model for the multiple high-order harmonic generation (HHG) phenomena in the tunneling limit. However, there are a number of subtle aspects of HHG such as lower order harmonics, threshold harmonics, fine structure in plateau harmonic peaks, and blue shift etc., cannot be explained by the quasiclassical model. Using precision time-dependent wave function obtained by the generalized time-dependent pseudospectral method( Xiao-Min Tong and Shih-I Chu, Chem. Phys. 217) (1997) 119., we have performed detailed time-frequency profile analysis of individual harmonic in various energy regimes for atomic H in intense pulsed laser fields using continuous wavelet transform^2. New insights on the subtle details of HHG mechanisms are obtained and the range of validity of the quasiclassical model assessed(Xiao-Min Tong and Shih-I Chu, Phys. Rev. A (submitted).).

[GP01.50] Exterior Complex Scaling - Generalized Pseudospectral Method for Complex Quasienergy Resonances: Application to Multiphoton Detachment of H^- Near the One-Photon Detachment Threshold

We introduce a new exterior complex scaling (ECS) method with generalized pseudospectral (GPS) technique for the determination of atomic and molecular resonances(D. A. Telnov and S.I. Chu, Phys. Rev. A (submitted).). The ECS-GPS procedure is numerically highly accurate and computational more efficient than the corresponding basis set -variational methods. It also provides a simpler procedure than the uniform complex scaling method for the calculations of partial rates and electron angular distribution. The ECS-GPS method is applied to the study of multi-photon detachment and electron angular distribution of H^- near the one-photon detachment threshold^1. The results are in good agreement with recent experimental data( L. Prastegaard, T. Andersen, and P. Balling, Phys. Rev. Lett. (submitted).).

[GP01.51] Time-Dependent Density Functional Theory With Optimized Effective Potential and Self-Interaction Correction for Molecular Multiphoton Processes in Intense Pulsed Laser Fields\

We develop a new time-dependent density functional theory (TDDFT) with optimized effective potential (OEP) and self-interaction correction (SIC) for ab initio\ nonperturbative treatment of multi-electron molecular multiphoton dynamics in intense pulsed laser fields(X. Chu and S. I. Chu, Chem. Phys. Lett. (submitted).). The theory overcomes the shortcoming of traditional DFT/TDDFT and provides an accurate description of both short and long range potentials, yielding accurate eigenvalues for both individual orbital energies and the ionization potential. The time-dependent wave function can be accurately and efficiently propagated by means of the generalized pseudospectral time-dependent procedure(X. M. Tong and S.I. Chu, Chem. Phys. 217) (1997) 119. on non-uniform grid lattice. The theory and method will be illustrated by the case study of multiphoton ionization, high-order harmonic generation, and Coulomb explosion of H_2 and N_2 molecules^1.

[GP01.52] Dark state splitting in 4-level system

Four-level system under four strong fields with frequencies to combine into a cycle is analyzed for double-\Lambda scheme. A new simple algebraic criterion of coherent population trapping is suggested. The dark state is found to split, i.e., arises at two values of the detuning, as opposed to the three-level case. The splitting is shown to manifest itself as two sharp dips in the frequency dependence of the upper level population; repulsion between the nonabsorbing states is demonstrated when Rabi frequencies exceed relaxation rates. Similar dips are found in nonlinear susceptibility, which is responsible for the conversion efficiency of resonant four-wave mixing.

[GP01.53] Intensity Dependence of the Plateau Structure in Laser-Assisted, X-Ray--Atom Scattering Processes.

The intensity dependence of the recently discovered (D. B. Milo\vs)evi\'c and F. Ehlotzky, Phys. Rev. 58, 2319 (1998). plateau structure in laser-assisted x-ray--atom scattering is analyzed for stronger laser fields. Using the ``three-step'' model and the strong-field approximation we show a connection between this process and high-order harmonic generation. For x-ray--atom scattering in the presence of a static electric field for higher laser field intensities a new plateau appears which is 5 orders of magnitude higher than the recently discovered (D. B. Milo\vs)evi\'c and A. F. Starace, Phys. Rev. Lett. 81, 5097 (1998). high-energy plateau. The position, as well as the height, of these plateaus is explained using semiclassical arguments. Supported in part by NSF Grant No. PHY-9722110.

[GP01.54] Control of High-Harmonic Generation with Parallel Static Electric and Magnetic Fields.

We consider the possibility of control of high-harmonic generation in a linearly polarized laser field by adding static electric and magnetic fields which are parallel to the laser polarization. We show that the magnetic field can considerably increase the harmonic intensity. A chosen harmonic is emitted with maximum efficiency for such values of the B field for which the classical period of the electron's motion perpendicular to the magnetic field is equal to the return time of the laser-field ionized electron wave packet to the nucleus. While the static magnetic field has only a small effect on the position of the cutoff, the static electric field can introduce additional plateaus and cutoffs. A properly chosen combination of the static electric and magnetic fields can increase both the harmonic intensity and the harmonic order. Supported in part by NSF Grant No. PHY-9722110.

[GP01.55] Polarization and Static Electric-Field-Induced Effects on Harmonic Generation (HG)

The general structure of the atomic dipole moment vector and nonlinear susceptibilities are analyzed for an arbitrary geometrical arrangement of an elliptically polarized laser and a static electric field. For an n-th order harmonic, we present its intensity, ellipticity, polarization ellipse orientation, elliptical and circular dichroism effects, and phase relative to the pump laser. We find that polarization effects in HG with and without a static electric field are very different. Particular effects discussed include static-field-induced HG by circularly polarized lasers and dissipation-induced ellipticity and circular dichroism by linearly polarized lasers. Quantitative numerical predictions of these effects and their dependence on appropriate geometrical and dynamical parameters are presented.

[GP01.56] Changes in Floquet-state structure at avoided crossings: Delocalization and harmonic generation.

We examine the sinusoidally driven particle in a square potential well to show that avoided crossings in the quasienergy spectrum can alter the structure of Floquet states in this system (T. Timberlake and L. E. Reichl, quant-ph/9810014). Two types of avoided crossings are identified: one type leads only to temporary changes (as a function of driving field strength) in Floquet state structure while the second type can lead to permanent delocalization of the Floquet states. Radiation spectra from these latter states show a significant increase in high harmonic generation as the system passes through the avoided crossing. Both avoided crossings and high harmonic generation in the quantum system can be associated with the spread of chaos in the classical system (W. Chism, T. Timberlake, and L. E. Reichl, PRE \textbf58), 1713 (1998). The delocalization that occurs at the avoided crossings leads, at high field strengths, to Floquet states that fill the classically chaotic region. The cutoff in the high harmonic generation is then determined by the energy width of the chaotic region.

[GP01.57] Light-induced Atomic States and Diabatic Paths in Intense Laser Fields

The physical reality of light-induced states (LIS) in atoms has remained uncertain, ever since their discovery by Floquet theory. We have shown that their existence is confirmed by time-dependent wave packet theory, and should manifest itself experimentally(J.C.\ Wells, I.\ Simbotin, and M.\ Gavrila, Phys.\ Rev.\ Lett.\ 80), 3479 (1998). By applying a realistic pulse to the atomic system, and calculating the energy spectrum of the ionized electrons, we find signals at the energies predicted by Floquet theory for the LIS, sometimes with towering intensity. Choosing the initial states such as to connect to the LIS via diabatic Floquet paths substantially enhances the yield. The physical reality of LIS has far reaching consequences for strong-field dynamics. After materialization, the LIS behave like any other Floquet state, and, together with the Floquet states which continue the stationary states of the atom, determine the spectral properties of the atom in the field. The LIS are a reservoir of states participating as intermediate steps in EPI/ATI, HHG, several-color excitation, and stabilization dynamics, and offer paths of evolution of the atom to super-high intensities.

[GP01.58] Angular Distributions of Photoelectrons in Above-Threshold Ionization

A calculation of the angular distributions of photoelectrons produced in above-threshold ionization has been performed according to a recent theory(J. Gao, D.-S. Guo and Y.-S. Wu, to be published), which extends the scattering theory of Guo, Åberg and Crasemann by inclusion of spontaneous emission, at the beam intensities with a noninteger ponderomotive parameter. The calculation shows qualitative agreement with experimental results.

[GP01.59] Inclusion of Electron Dynamics in a 3D Global Gyrokinetic Particle Model

Recent experimental results have underlined the importance of the electron channel in understanding the tranport of particles and heat at the core of present day tokamaks. We have embarked on a program which aims at including electron dynamics in our three-dimensional global electrostatic gyrokinetic code used to model ion temperature gradient driven turbulence (ITGDT) but with adiabatic electrons. A drift kinetic, guiding center description of the electrons, including mirroring, has been deemed adequate and is being implemented using a delta-f framework which is also used for the gyrokinetic ions. A time-implicit treatment has been adopted to avoid the fast time scales, hence small time steps, associated with the motion of the electrons parallel to the magnetic field. In addition to exploring the properties of delta-f descriptions for more than one species of particles, applications of this massively parallel modeling tool, will range from ITGDT to current self-generation and sustainment.

[GP01.60] Effect of Magnetic Shear Reversal on the Parallel Velocity Shear Instability

The nonlinear behavior of the parallel velocity shear instability in a sheared magentic field is studied. It is found that when the magnetic shear as has the same sign as the second radial derivative of the parallel velocity profile, the instability is destabilized even further than if there were no magnetic shear. When the magnetic shear is reversed, the instability is considerably weaker. This effect is consistent with the observed reversed shear stabilization observed in tokamaks for the case of field-aligned flows. A physical mechanism is presented that explains this phenomenon, and it is shown that this effect may be a common feature of all instabilities with a mode structure both the parallel and perpendicular to the magnetic field.

[GP01.61] A Multi-Grid Algorithm for Nonlocal Collisional Electrostatic Drift-Wave Turbulence

We have developed a three-dimensional anisotropic multi-grid solver for simulating nonlocal collisional electrostatic drift-wave turbulence in the presence of magnetic shear. This solver has been used to obtain entire flux surface solutions of the nonlocal Hasegawa--Wakatani equations. In contrast with flux-tube models, which necessarily require approximation of the parallel boundary condition, the solution of a full tokamak flux surface in field-line coordinates allows the doubly periodic boundary condition to be expressed without approximation. The implicit treatment of the parallel-gradient terms permits the use of a relatively large time step. Considerable effort was made in the design of the implicit solver to ensure that the presence of anisotropy does not lead to a significant degradation in performance. The multi-grid algorithm has several advantages over a pseudospectral Poisson solver; most importantly, all nonlinear terms, including those in the Ohm's law, can be retained in a straightforward manner. Although in this work the multigrid method is illustrated using straightened tokamak geometry, the object-oriented construction of the code will facilitate the eventual inclusion of curvature terms and the complete nonlinear reduced Braginskii equations, including ion thermal dynamics.

[GP01.62] A Dynamical Model of "Avalanches"

The currently popular self-organized criticality paradigm rests on the concept of an avalanche. In a cellular automaton model, an avalanche is simply a coherent chain of topplings. In plasma dynamics, avalanche concept suggests the notion of an "event", where several adjacent modes collectively act to drive a burst of enhanced transport. "Transport events" or "avalanches" have been observed in numerical simulations, but the dynamics of avalanches remains obscure.

Here, we propose that the "avalanche" is a collective excitation in the gas of waves, - i.e. a radially extended, poloidally thin modulational wave or instability. Thus, an avalanche is, effectively, a streamer. We develop this proposal by examining the modulational stability of streamers in a gas of interchange - thermal Rossby modes. A streamer stability criterion and growth rate have been calculated. Streamers/avalanches are excited by both a turbulent Reynold's stress and by turbulence - enhanced pressure. The back-reaction of streamer events on the underlying turbulence is the subject of an ongoing study, as is the interplay between streamers and zonal flows.

[GP01.63] Self-Organizied Criticality and Force-Free States

Here, we examine force-free magnetic field states in a bath of noise. In terms of the transport SOC paradigm(D. E. Newman, et. al., Phys. Plasmas 3), 1858 (1996). the marginal profile is the force-free state, known to be marginal to tearing. The conservation law is that of magnetic helicity. Enforcing minimum energy determines the marginal profile. Deviations from marginality can be represented by an equation for the force-free parameter (ratio of parallel current to magnetic field), or an equation for helicity transport. These deviations can be generated by resistive dissipation, and the fluxes can be represented by alpha and beta effects, as well as the novel fluctuation generated hyper-resistivity (analogous to water running down an inclined plane.) In analogy with transport paradigms, SOC states can be sub-marginal, which for force-free marginality, results in a lowering of the minimum of the minimum energy state.

[GP01.64] Fundamental Transport Suppression Mechanisms arising from Reversed Magnetic Shear and Radial Electric Field Shear

A fundamental understanding of transport barriers in tokamaks is presented in terms of drift wave maps which are sympletic representations of the guiding center differential equations for the charged particle motions in tokamaks with fluctuations. Ensembles of test particles launched from the core of the tokamak are advanced forward in time to determine the effect of various rotational transforms profiles on the global particle confinement. The global confinement time is show to be enhance for particles that have a shearless winding number in their map. The winding number for electrons depends principally on the magnetic field rotational transform and shows enhanced confinement for Reversed Magnetic Shear Profiles (RS) plasmas. The winding number of the ions depends on both the magnetic-q profile and the rotation from the radial electric field. Thus, the confinement enhancement for ions can be strongly improved by deep wells in the radial electric field profile. Neoclassical theory is used to calculate self-consistient profiles of Er for the both monotonic and reversed magnetic shear profiles for particular TFTR discharges. The orbits of the full guiding center and those from from the sympletic maps are different as shown in detail. The ensembled averaged global confinement properties from the two approaches are thought to track one another. A third approach using reductions two low-order moments from stochastic differential equations that include the effect of the Coulomb collisional scattering of the charged particles is also compared with the drift wave map confinement predictions.

[GP01.65] Kirchhoff Thermal Radiation - The Cause Behind Anomalous Transport

The principal toroidal transport anomalies, namely, (i) enhanced particle and thermal diffusivity, (ii) magnetic-field dependence, (iii) temperature dependence, (iv) density dependence, (v) isotope-mass dependence, and (vi) power degradation in auxiliary heated plasmas, are all traced to momentum-exchange collisions induced via Kirchhoff thermal radiation(S. Puri, Phys. Plasmas \bf5), 2932 (1998). For the high-frequency Bernstein modes responsible for radiative collisionality, the product of frequency and plasma parameter greatly exceeds the two-particle collision frequency; the expansion procedure in terms of the plasma parameter used in the Balescu-Lenard type of equations is not applicable. Thus, Kirchhoff's law is the only presently available method for determining radiative collisionality. The simple yet rigorous approach allows important insights into the basic mechanisms responsible for toroidal transport and possesses definite predictive capability for reactor-scale plasma performance.

[GP01.66] The gyro-radius scaling of ion thermal transport from global numerical simulations of ITG Turbulence.

A three-dimensional fluid code is employed to study the scaling of ion thermal transport caused by Ion-Temperature-Gradient-Driven (ITG) turbulence in a tokamak, and in particular the dependence of the effective ion thermal conductivity \chi_i on the reduced gyroradius \rho_\displaystyle * \equiv \rho_s/a, where \rho_s is the ion sound Larmor radius and a the machine minor radius. The code includes toroidal effects and is capable of simulating the whole torus. It is found that both close to the ITG threshold and well above threshold, the thermal transport exhibits the gyro-Bohm scaling \chi_i / \chi_B \sim \rho_\displaystyle *, where \chi_B is the Bohm diffusivity, at least for plasmas with moderate poloidal flow. This result is confirmed by the analysis of some quantities that characterize the turbulent fluctuations (correlation lengths and time and the fluctuation level), which also exhibit gyro-Bohm scaling.

[GP01.67] Mean Field Transport Equations Including E x B Velocity

The stabilization of turbulence by E\times B velocity shear has been incorporated into models of tokamak transport in a number of forms. The radial electric field is determined by the perpendicular and diamagnetic velocities through the radial force balance for the ions. Thus, the E\times B velocity shear involves the second derivative of the ion pressure. This make the system of transport equations of third order in the pressure gradient. It has recently been pointed out that the solution one obtains can be stongly influenced by the choice of boundary condition for the second derivative of the pressure.(J.B. Taylor et al.), Phys.\ Plasmas 5 (1998) 3065. The choice of this boundary condition is undertermined by the physics. In this paper a systematic derivation of a set of mean field transport equations is presented. The turbulent fluxes are averaged over the instability time and length scales. The fluxes only involve first derivatives of the fields. An equation for the evolution of the E\times B velocity is derived. Physically motivated boundary conditions exist for all of the fields. The poloidal velocity is determined from radial force balance. The poloidal velocity can strongly deviate from its standard neoclassical value due to the perpendicular turbulent viscosity.

[GP01.68] Parallel Velocity Shear Driven Modes in Discharges with Reverse Magnetic Shear

The linear and quasilinear behaviour of the short-wavelength drift-like perturbation with a parallel velocity shear is studied in a sheared slab geometry. Full analytic studies show that when the magnetic shear has the same sign as the second derivative of the parallel velocity with respect to the radial coordinate, the linear mode may become unstable and turbulent momentum transport increases. On the other hand, when the magnetic shear has opposite sign as the second derivative of the parallel velocity, the linear mode is completely stabilised and turbulent momentum transport reduces. This result therefore shows that it is the relative sign of the second radial derivative of the equilibrium parallel flow with respect to the magnetic shear which is the key factor for the enhance reverse shear transition.

[GP01.69] Local Scalings and Nonlocal Effects in Toroidal ITG Turbulence

We report new results on toroidal ITG turbulence from nonlinear gyrokinetic simulations. (A) For some ion temperature gradient scans, the local ion thermal flux \Gamma is found to be very accurately described by a form \Gamma \propto \partial_r T - \partial_r T|_eff, where \partial_r T|_eff is an effective critical temperature gradient that is larger than the linear critical temperature gradient. The excellence and simplicity of the fit, including the ability to obtain \partial_r T|_eff by extrapolation from values of \Gamma with \partial_r T > \partial_r T|_eff, suggest a considerable simplification of the underlying physics. The robustness of this dependence to various physical effects and parameter variations will be explored.

(B) A new analysis method, based on scatter plots of radially local time-window-averaged thermal flux vs.\ gradient, has been applied to explore whether the transport in global gyrokinetic simulations is local. For profile variation scales characteristic of DIII-D, the transport is found to be nonlocal; there are regions of low and high temperature gradient where the transport is respectively significantly larger and smaller than the local prediction. A sequence of larger simulation runs is underway to investigate when a transition to local behavior occurs.

[GP01.70] Negative-energy perturbations and reversed-magnetic-shear transport barriers in tokamaks

The impact of reversed magnetic shear (s<0) along with a radial electric field E_r on negative-energy perturbations (NEP's) is investigated for the case of cylindrical tokamak-like equilibria. NEP's can imply instabilities leading to anomalous transport [G. N. Throumoulopoulos and D. Pfirsch, Phys. Rev. E 56, 5979 (1997) and Refs. therein]. For equilibria with E_r=0 and \eta_\nu<4/3 (\eta_\nu\equiv \partial\ln T_\nu /\partial\ln N_\nu, and \nu denotes the particle species) s<0 results in a reduction of the fraction of particles associated with NEP's (active particles) in comparison with that for equilibria with s>0, by making the pressure profile steeper. The reduction is stronger the closer the minimum of the safety factor (q_\min) to the plasma center and the lower the negative value of s. For equilibria with E_r < 0 a reduction of the fractions of active ions for \eta_i<4/3 and of active electrons for \eta_e>4/3 is caused by E_r irrespective of the sign and the value of s and of the position of q_\min. The present results indicate that the effect of s<0 on NEP's is not additive to that of E_r. Also, it is shown that the experimentally evidenced facts that (a) s<0 is associated with a steepness of the pressure profile and (b) the position of q_\min is displaced towards the plasma edge as the toroidal current is increased are consistent with equilibrium considerations.

[GP01.71] Electromagnetic, Nonlinear Gyrokinetic Simulations

A widely used, comprehensive linear gyrokinetic code(Kotschenreuther, M., et al.,) Comp.~Phys.~Comm., 88, 128 (1995). has been generalized to be both massively parallel and nonlinear. The simulation domain is a flux tube set in general toroidal geometry. This code evolves distribution functions for electrons and an arbitrary number of ion species in time on a grid in three spatial dimensions, energy, pitch angle, and the sign of the parallel velocity. Trapped particle dynamics are fully included, and collisional effects are treated with a Lorentz collision operator. In each direction, non-uniform coordinate grids are chosen to maximize resolution where needed. A simulation domain of 48*96*96*10*60*2 uses 500 million grid points for each species. Thus, good parallel performance is required. To attain this goal without compromising portability, we have completely rewritten the original code using object oriented methods; the resulting code runs correctly on multiple MPP platforms. Available computational and physics results will be presented.

[GP01.72] Effective Field for Turbulent Transport in Shaped Tokamak Plasmas

The nonlinear ballooning mode code(R.E. Waltz et al.), Phys.\ Plasmas 1 (1994) 2229; ibid.\ 5 (1998) 1784.\ for ITG turbulence now has a generalization(R.L Miller et al.), Phys.\ Plasmas 2 (1998) 973.\ of the circular \hat s-\alpha local MHD equilibrium model to finite aspect ratio (A), elongation (\kappa), and triangularity (\delta). This allows us to make systematic studies of shaped flux surfaces with the same minor midplane radius lable (r), plasma gradients, q, \hat s, and \alpha, while varying A, \kappa, and \delta. We show that the (linear, nonlinear, sheared) E\times B terms in the equation of motion are unchanged from a circle at radius r with an effective field B_unit=B_0\rho d\rho/rdr where \chi=\rho^2/2 is the toroidal flux, and r is the flux surface label. This leads to a ``natural gyroBohm diffusivity'' \chi^natural which at moderate q=2--3 is weakly dependent on shape (\kappa) at fixed B_unit. However since B_unit/B_0\approx\kappa (or larger), the label independent \chi^ITER=(B_0/B_unit)^2\chi^natural/ \langle(\nabla r)^2\rangle at fixed B_0 scales as 1/\kappa^1.6 (weaker scaling at high-q and stronger at low-q). Linear stability is not a good indicator of scaling with \kappa. The generalized E\times B shear rate to be compared to the maximum linear growth rate^2 is a flux surface quantity (r/q)d/dr(cq/rB_unitd\phi_0/dr) =(r/q)d(cE_x0/B_\theta R)/dr.

[GP01.73] Turbulence Simulation Using A Gyrokinetic Ion-Fluid Electron Hybrid Model

We describe the formulation of a hybrid model with fully gyrokinetic ions and zero-inertia fluid model for electrons. The electron fluid equations are derived from moments of the drift kinetic equation, taking m_e\rightarrow0, but with T_\parallel e finite, thereby avoiding all accuracy or stability constraints on k_\parallel v_te \Delta t, as well as particle noise associated with electron free streaming. This approach is a natural extension of the electrostatic gyrokinetic simulations to include effects of electron ExB flow, electron pressure gradient effects (e.g. ømega_*e), and most importantly the electron parallel current, which in turn is used to include electromagnetic perturbations perpendicular to the equilibrium B-field (\delta B_\perp). A nice property of this model is that it includes the shear Alfvén wave physics while eliminates high frequency waves associated with the electron inertia, and is therefore suitable for studying the \beta! d! ependence of ITG driven microins tabilities (including the recently discussed Alfvén ITG(Zonca et. al.), Bull. Am. Phy. Soc. 43(8), 1921(1998)). A predictor-corrector scheme for the hybrid model has been developed for the simulation of a flux-tube. Details of this integration scheme will be presented, with an emphasis on benchmarking the shearless slab result aginst the local dispersion relation. Preliminary results for a toroidal flux-tube will also be presented and discussed.

[GP01.74] 3D Simulations of Tokamak Edge Turbulence in General Geometry

In past work(B. N. Rogers, J. F. Drake, A. Zeiler, Phys. Rev. Lett. 81), 4396 (1998) we described a model for the LH transition and density limit in tokamaks based on 3D flux tube simulations of edge turbulence. The simulations were based on the the Braginskii equations and assumed a simple shifted circle magnetic geometry. At a qualitative level, the predictions of the model show encouraging agreement with experimental data in CMod, DIIID, and ASDEX-U. Quantitatively, however, substantial uncertainty arises in the comparison of the model to experiments due to the absence of realistic magnetic shaping effects in the simulations. We report on here our progress toward including such shaping effects into our 3D simulations of edge turbulence and the impact these effects have on the simulated anomalous transport.

[GP01.75] Thermal Transport Coefficient for L-Mode Discharges

Starting from an analysis of the transport properties of the plasmas produced by the Alcator C-Mod machine, a novel form of a thermal transport coefficient(B. Coppi and W. Daughton, in 24^th) EPS Conference on Controlled Fusion and Plasma Physics, (1997) which reproduces both ohmic and auxiliary heated L-mode discharges has been identified. This thermal transport coefficient involves the difference of two terms, one representing the normal outward diffusion of thermal energy and the oth er corresponding to a process reducing the outward flux. The coefficient includes the constraint of profile consistency and is inspired by the properties of the toroidal ion and electron temperature gradient modes. In a series of transport simulations for plasmas produced by the Alcator C-Mod machine, the thermal transport coefficient is shown to reproduce the observed electron temperature profile, stored energy and loop voltage. The relevant scaling for the energy confinement time is also composed of t wo terms. The L-mode database assembled for the ITER project is used to compare DIII-D, JET, JT60, PDX, TFTR, FTU, and Alcator C-Mod with this analysis. With the exception of PDX, the comparison is substantially better than standard L-mode pow er law scalings.

[GP01.76] 3D Global Gyrokinetic Particle Simulation Study of Turbulence Suppression in Neon Impurity-Seeded Tokamak Plasmas

Recent experiments have shown that improved confinement regimes of tokamak plasmas can be obtained through a controlled introduction of a radiating impurity such as Neon or Argon. There is also evidence that turbulent fluctuations are being suppressed as the impurities are being distributed. We have been investigating the physical processes involved in turbulence suppression under these conditions using experimentally determined temperature, density, and magnetic field profiles taken before and after impurity injection in the TEXTOR-94 tokamak plasma. We use a nonlinear 3D global toroidal gyrokinetic particle simulation model which contains multiple ion species (R.D. Sydora, V.K. Decyk and J.M. Dawson, Plasma Phys. Contol. Fusion, 38,p.281,(1996). Self-generated turbulent zonal flows from flux surface-averaged potentials as well as equilibrium sheared ExB flows are included in the model. Preliminary results indicate that suppression of toroidal ion temperature gradient instabilities occurs as a result of density peaking from reduced particle transport and direct stabilization of ITG modes via dilution and impurity gradient effects. Scaling studies with various parameters such as impurity concentration and gradient will be presented.

[GP01.77] Split-Weight \delta f Gyrokinetic Particle Simulation Scheme for Finite-\beta Plasmas

A split-weight \delta f gyrokinetic particle simulation scheme for finite-\beta plasmas have been developed. This is an extension of the electrostatic version of the split-weight scheme^1. In this scheme, the standard \delta f response is splitted into an adiabatic part and an nonadiabatic part, with the latter followed dynamically for each particle. Since the phase velocities for the waves of interest, such as shear-Alfven waves, are usually much smaller than the electron thermal speed for \beta \gg m_e/m_i, the nonadiabatic responses for most of the electrons are nearly zero and do not contribute to the velocity moments. Thus, the parallell Courant condition of k_\parallel v_\parallel \Delta t \ll 1 for the fast particles can be circumvented resulting in considerable increase in time steps for the code as well as the reduction of noise from the fast moving particles. The use of the split-weight scheme based on the generalized Ohm's law^2 as well as the p_\parallel formalism^3 for studying shear-Alfven waves and their coupling with drift waves and ion temperature gradient modes will be presented.

^1 I. Manuilskiy. W. W. Lee, and H. Mynick, Bull. Am. Phys. Soc. 43, 1723 (1998). ^2 J. Reynders, Ph. D. thesis, Princeton University, 1992. ^3 T. S. Hahm, W. W. Lee, and A. Brizard, Phys. Fluids 31, 1940 (1988).

[GP01.78] Gyrokinetic Solver on Linux Beowulf

Progress on a parallel, full-radius, electromagnetic, gyrokinetic, Eulerian (fluid-like) code is detailed. In particular, we emphasize the inclusion of passing electron dynamics required for electromagnetic perturbations. The electron parallel motion is treated using implicit numerical methods, so that the requisite short time-step of any explicit method can be avoided. We hope to elucidate the role of profile effects in plasma transport using a formalism free of uncertainties related to fluid closures, particle noise, and the restriction to electrostatic perturbations.

The development platform is a 16-node parallel distributed memory Linux Beowulf cluster supporting MPI interprocessor communication. Such a developent environment is ideal for the production of comparitively stable, well-documented, bug-free, and portable code. We expect this code to migrate easily to future TFLOP machines which provide the vast computational resources necessary for production runs.

[GP01.79] A Framework for Gyrokinetic Particle Simulation of Electromagnetic ITG Turbulence

There has been a recent growth in interest in electromagnetic effects on tokamak plasma turbulence and anomalous density and energy transport. Earlier studies of linear ion-temperature-gradient (ITG) driven instabilities have indicated that the electrostatic ITG mode is strongly suppressed by finite plasma beta. More recently, it has been theorized that Alfvén modes in the continuum gap may be discretized by finite-Larmor-radius (FLR) and finite-orbit-width (FOW) effects and driven unstable at a plasma beta well below the ideal MHD critical value. Furthermore, experimental results indicate disparate behaviors in the ion and electron thermal transport channels in response to certain changes in plasma conditions, which may be an indication that electromagnetic perturbations are playing a more important role in electron transport than previously thought. Thus, the need for intensive study and simulation of electron dynamics and electromagnetic turbulence has never been greater. In order to address these issues, we propose a general framework for gyrokinetic particle simulation of electromagnetic perturbations. This framework will be structured in a way that will allow various models for different plasma species, geometries, perturbative quantities and plasma diagnostics to be easily interchanged in a ``plug-and-play'' environment. In this way, we can use the same code base to simulate Alfvénic ITG modes, energetic particle modes, and trapped-particle effects, all with a choice of models for the electron dynamics. We describe here the workings of our framework approach and how it allows us to leverage our software development and attack several interesting physics problems at once.

[GP01.80] Coupling Transport and Turbulence in Edge-Plasma Simulations

The collisional edge region of a tokamak is often described by fluid equations for plasma density, parallel velocity, electron and ion temperatures, and electrostatic potential. Transport codes such as UEDGE use empirical diffusion coefficients for transport across the magnetic field, B, to evolve these variables in 2-D over long space and time scales to obtain equilibrium profiles. Turbulence codes such as BOUT typically use fixed profiles to calculate the 3-D turbulence on shorter space and faster time scales, yielding physics-based cross-field fluxes or diffusion coefficients. We present initial results of an iterative coupling between UEDGE and BOUT over the pedestal gradient edge-region and the scrape-off layer outside the magnetic separatrix giving a self-consistent simulation. Of particular importance is the generation of a strong radial electric field with rapid radial variation which produces a sheared E \times B flow that helps suppress turbulence, thereby creating an H-mode transport barrier in the edge-region. The dynamic evolution of plasma profiles and turbulence is necessary to identify the trigger mechanism for L-H mode confinement transitions. An assessment is given of the computational resources required to perform these coupled simulations on a routine basis.

[GP01.81] Progress in Nonlinear Gyrokinetic Simulation

Progress in nonlinear gyrokinetic simulation of turbulent transport in tokamaks is reported. We are porting our general noncircular-cross-section equilibrium capability(A.M.\ Dimits, Bull.\ APS 41), 1413 (1996). to our parallel (T3E) flux-tube gyrokinetic code. Initial results will be shown using both reconstructed DIII-D equilibria and equilibria from MHD equilibrium solvers, focusing on benchmarks and convergence tests made possible by the the implementation of this physics on larger computers. A new bounce-averaged drift-kinetic \delta f electron algorithm has been formulated and is being implemented in our gyrokinetic code. Linear benchmarks and initial studies addressing the effect of nonadiabatic electrons on ITG turbulence and transport will be presented. Results of continuing studies of the convergence of our gyrokinetic simulations with respect to particle number, grid size, and system size, as well as dependence on initial conditions will also be presented.

[GP01.82] A nonlinear correction to Landau-fluid closures suitable for gyrofluid turbulence simulations

An simple to use method for calculating the nonlinear correction to a given gyro-Landau-fluid closure term is presented. In order to calculate the nonlinear corrections, it is assumed the a reasonable linear closure term exists. This seems to be the case for existing gyrofluid equations which are fairly accurate linearly. In addition, perturbation theory is used, requiring the fluctuations to be small compared to the equilibrium values which is generally the case for drift-wave type tokamak turbulence. This approach introduces one more quantity to time evolve, but makes no reference to ømega or k_\perp, and is therefore well-suited for current pseudo-spectral simulations.

[GP01.83] Field-line Coordinates for Global Gyrokinetic Simulations

The field-line following magnetic coordinates has been implemented in the global gyrokinetic toroidal code (GTC). This enables us to take advantage of long parallel wavelength of microinstabilities believed to be responsible for transport in magnetized fusion plasmas. As a result, computational cost of a global simulation is dramatically reduced, and routine large scale simulations for the study of key issues in turbulent transport become possible. Our GTC code simulations have demonstrated decorrelation of nonlinearly saturated turbulence, and reduction of associated transport by self-generated zonal flows^1. Further studies show that ion-ion collisions can regulate transport via zonal flow damping. Close to marginal stability, turbulent damping of zonal flows is weak, and the saturation of the flows and thus the turbulent transport can depend on collisionality. Since effects of collisions have been neglected in most turbulent transport studies of high temperature plasmas, our results provide new insight into the transport scaling. Comparisons of simulations with analytic calculations^2 of the collisional flow damping will also be presented. \vspace0.1in

^1 Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, and R. B. White, \hspace*0.1inScience 281, 1835(1998). ^2 F. L. Hinton and M. N. Rosenbluth, Proc. of the 25th Euro. Conf. \hspace*0.1inon Contr. Fusion and Plasma Phys., Praha, Czech Republic (1998).

[GP01.84] Effects of Temperature Ripples and Self Consistent Equilibrium E_r Shear in a Gyrokinetic Flux-Tube Simulation

Gyrokinetic simulations of ion-temperature-gradient-driven turbulence have shown the equilibrium E_r shear suppression of core ion heat transport. In addition, it has been shown that self-generated E_r shear is an important stabilizing effect, especially in flux-tube simulations. The present simulation includes all effects of pressure profile variation, including the equilibrium E_r shear from radial force balance, as well as the usual self-generated E_r shear. The self-generated purely radial electrostatic porential has been observed to be 180^o out of phase with the flux-surface-averaged perturbed perpendicular temperature. This correlation explains the different shapes of the purely radial modes found in various global and flux-tube simulations. Based on these observations, a novel method to reduce the ion heat transport is proposed. By introducing a temperature ripple as small as 2%, on a spatial scale of 30\rho_i, simulations have shown 60% reduction in the heat flux. However, these preliminary results did not include equilibrium E_r effects. It is possible that for some parameter values, these effects will cancel one another. We will present results that self-consistently include equilibrium E_r shear, as well as, a more detailed study of the parameters involved in the ripple transport suppression mechanism.

[GP01.85] A Theory of Edge Convection

Recently, a set of nonlinear model equations\footnote D.A. D'Ippolito, J.R. Myra, V. Bhatnagar, J. Jacquinot, Bull. Am. Phys. Soc. 43, 1755 (1998). was derived describing the interaction of driven convection with the tokamak edge plasma electric field and electron temperature T_e. For collisional plasmas described by the Braginkii equations, it was shown that strong convection could produce significant cooling and a reduction in the E \times B shear. The convection can be driven by a poloidal modulation of either the edge potential \phi (e.g. by ICRF heating) or the edge T_e (e.g. by gas puffing). Here, we extend the theory by solving the model equations numerically to obtain the scaling of the penetration of the convection. This work is motivated by experimental results on JET showing that 1) H-mode properties such as the ratio of \tau_E/\tau_p, the temperature pedestal height, and the ELM amplitude and repetition rate can be significantly different for ICRF H-modes under certain conditions (e.g. for low k_\parallel antenna phasing) than for NBI H-modes; and 2) there is an interesting parallel between the effects of ICRF and gas puffing on the H-mode. The relevance of this theory to JET and C-MOD experiments will be discussed.

[GP01.86] Resistive X-point modes in boundary plasmas

In recent work(X.Q. Xu et al., presented at the 17th IAEA Fusion Energy Conf., Yokohama, Japan, October 1998, paper IAEA-CN-69/THP2/03.) we have identified the curvature-driven resistive X-point mode as the dominant instability of a characteristic L-phase discharge in DIII-D. This mode, expected to be a generic L-mode edge/SOL instability, is electromagnetic near the outboard midplane, but transitions to an electrostatic mode near the X-point due to the combined effects of resistivity and X-point magnetic shear. Motivated by observations of elevated electron temperature near the X-point,(M.J. Schaffer et al., Bull. APS 43, 1889 (1998), paper R8P5.) we investigate heating and parallel energy flow induced by the resistive X-point mode. We speculate that energy in the unstable waves flows to and dissipates in the X-point region, heating the electrons. The mode is also expected to dominate perpendicular wave-induced transport. Modifications of the usual mixing length estimates to take into account the X-point geometry are shown to be required.

[GP01.87] Analysis of Anomalous Growth in Core Electrostatic Turbulence

Recent observations of large electron thermal transport in the flat temperature profile region of enhanced reverse shear (ERS) discharges suggests that nonlinear instability may play a role. Simulations of a two-field fluid collisionless trapped electron mode (CTEM) model show anomalous energy inputs and outputs at saturation which are localized in wavenumber but are not predicted by linear theory alone. This includes a mechanism for nonlinear anomalous energy growth. Prior results suggest that this is due to nonlinear excitation of a damped branch of the linear solution which is nonorthogonal to the growing branch, producing a frequency-dependent contribution to energy evolution. Through a variety of analytic techniques, including parametric excitation of the damped branch, iterative inversion of the nonlinear electron response, and analysis of nonorthogonal energy components, we will attempt to develop a deeper understanding of this effect and to predict and quantify it. We will also assess its role in an ion mixing mode (IMM) turbulence model, a variation of ion temperature gradient (ITG) turbulence which takes into account a nonadiabatic electron response.

[GP01.88] Impurity radiation and bursty plasma transport model

Impurity radiation (IR) plays an important role in the physics of edge plasmas. In "laminar" diffusive approach cross field plasma transport was shown to be one of the key element determining IR loss [1].However, there are indications that plasma transport may be far from "laminar" diffusion and is characterized by large bursts. We study nonlinear plasma transport model dT/dt=dDdT/dx/dx+S-R(T), where x is the spatial coordinate, T(x,t) is the plasma temperature , S and R(T) describe the heat source and IR sink, D=|dT(t-tau)/dx|**alfa is the plasma heat diffusivity, alfa is an adjustable parameter and tau a small time scale which can be interpreted as inverse growth rate of the plasma instability which result in nonlinear plasma heat diffusivity D. Tau though being very small causes the fine structures in our transport model to be unstable. As a result, transport becomes bursty and resembles SOC features. We report on the effects of our bursty transport model on IR loss and compare it with "laminar" transport models. [1] S. I. Krasheninnikov, Phys. Plasmas 4 (1997) 3741.

[GP01.89] 2-D Modeling of Ionization-Recombination Instability in the Edge Plasma*

Recent experimental findings in Alcator C-Mod tokamak [1] reveal that plasma strongly recombines in the MARFE region. Analysis of 0-D [2] and 1-D [3] models describing particle and energy balances shows that MARFE could be a non-linear stage of an ionization-recombination instability of hydrogen plasmas similar to that of a detachment front [4]. Based on a linear analysis of the instability for a slab geometry some 2-D model was built, allowing us to investigate numerically conditions for such an instability to appear. Here we report the results and compare our findings with the results of 0-D, 1-D models and experimental data. [1] B. Lipschultz et al., PRL (1998). [2] S. I. Krasheninnikov, et al., EPS-98. [3] A. N. Simakov, et.al., APS-98. [4] S. I. Krasheninnikov, et al., PSI-98.

[GP01.90] Kinetic Simulation of Coupled Plasmas and Neutral particles in the Scrape-off layer (SOL) of TdeV and in linear plasmas experiments.

In interesting detached regimes of TdeV, when the plasma detaches from the limiters, Coulomb Kudsen number increases with the local plasma gradients in an inhomogeneous magnetic field. Neutrals, which are being produced due to plasma recombination at the plasma-divertor interface, are also in a mixed collisional regimes. Thus, simultaneous kinetic treatments of plasma and neutral particles with self-consistent calculation of boundary conditions at the material walls are required. In linear machines, mirror effect due to inhomogeneous magnetic field and neutrals effects are very important. We present results for a kinetic simulation of plasmas and neutrals for actual conditions in TdeV and in a linear machine, where the effects of gradients and inhomogeneous magnetic field are taken into account.

[GP01.91] National Transport Code Collaboration (NTCC) Overview

NTCC is a multi-institutional collaboration whose goal is to change the way fusion modeling codes are constructed and used. The project includes extracting physics modules from existing codes and developing new modules. A review process has been developed to ensure that these modules satisfy well-defined standards. The accepted modules are placed in a Web-based community-owned NTCC module library. Modern computing techniques are used to construct a flexible framework, utilizing library modules, for the generation of easily maintained, customized, user-friendly transport codes that can be used to study new physics. This approach facilitates code use by non-experts and the examination of new transport models. Desktop access is provided through a browser to experimental data and to a physics server that can be located at remote sites. Data accessors enable examination of data in either U-file or MDS+ format. With the demonstration code that has been produced, the Multi-Mode, GLF23, IFS/PPPL, and OHE transport models can be compared and tested against experimental data.

[GP01.92] National Transport Code Collaboration Module Library

A library of computer code modules has been developed for the National Transport Code Collaboration (NTCC). Each module is an isolated, self-contained piece of software that is designed to carry out a specific function, such as the computation of the transport fluxes from a given model. After each module is submitted, it is evaluated by the Module Library Committee according to a list of standards. These standards require, for example, that each module is thoroughly documented and is provided with a stand-alone driver and test cases that compile and run correctly on different computer platforms. The resulting modules are intended to be easily inserted into transport codes and are available for anyone to use. Modules, standards, and additional information can be found on the Web page http://w3.pppl.gov/NTCC.

[GP01.93] Structure of the Demonstration Code for the National Transport Code Collaboration

The key concepts in the NTCC transport code are the use of plugable tokamak physics and numerical solution modules, steerable computations, remote invocation across the internet, interactive visualization of results, and automated retrieval of data using network transfers from remote MDSPlus data servers and Ufiles. The demonstration code features a physics server that is activated remotely within a standard web browser through application of CORBA technology. A Java-based gui client was developed that permits selection and interaction with various runtime control parameters and confiment models (GLF23, IFS\slash PPL, Multi-Mode, and OHE) and allows interactive graphical display of the resuslts as they are generated.

[GP01.94] Neutral Atom Transport Benchmark Studies

A validation study is being performed for the Transmission/Escape Probability (TEP) neutral atom transport method (W. M. Stacey, J. Mandrekas, Nucl. Fusion) 34 (1994) 1385., (W. M. Stacey, Phys. Plasmas) 4 (1997) 179.. The analytic formulation of the basic transport elements--the first-flight transmission and escape probabilities for a region--are being tested by comparison with exact results for slab geometry and by comparison with Monte Carlo for a range of geometries and region dimensions, and a new rational approximation is being developed for the escape probability. The accuracy of using of an average mean-free-path (mfp) to characterize a non-uniform region, which is done in the calculation of cumulative transmission and escape probabilities, is being tested. The isotropic incident flux assumption used in calculating transmission probabilities has been tested for multiregion transmission problems for regions of varying mfp, and an improved algorithm has been developed. Use of the local ion temperature to compute neutral mfps has been tested for non-uniform multiregion problems, and an iterative improvement is being developed. A multigroup formulation has been developed and will be implemented and tested. Accurate calculations of detailed scrape-off layer/divertor models of DIII-D, C-MOD and ITER require less than 1 min. of CRAY C90 time.

[GP01.95] Status of Physics Server for the National Transport Code Collaboration (NTCC) Demonstration Project

The NTCC demostration project utilizes modern software methodologies to develop a transport code that is easy to access, use, modify and maintain. We report on the structure and status of the physics application, presently a code which advances electron and ion pressure equations with density, flow, source and geometry profiles read in from a database. Modules for transport coefficients (typically Fortran) are wrapped and treated as class instances in an object-oriented heirarchy. Transport models available include IFS-PPPL, GLF23, and Multi-Mode. New modules, (e.g., neoclassical tranport, MHD equilbrium) will be added as they are made available through the NTCC Physics Module Library. We report on: the status of module installation; inter-model, inter-code, and data comparisons; tests of convergence of the models with various iteration schemes, physics complications (e.g., shear flow) and spatial representations; sensitivity to boundary conditions; and plans to upgrade to a fully predictive transport code.

[GP01.96] A Possible Confinement Time Barrier For Magnetic Fusion

One perplexing feature of the magnetic fusion program is the recent slow advance of the approach to "break even". One possibility is that there is a hidden physical limitation rather than the presently assumed engineering limitation. A past example of such a physical limitation is the scattering lifetime of the charged particles in the magnetic mirror machine being comparable to the break-even lifetime of Lawson's Criterion. An analogous limitation in present magnetic fusion devices may be simply black-body radiation. Any object that strongly absorbs radiation in a given frequency range must re-emit radiation as a black-body in the same frequency range. Let us make the simple assumption that a magnetically confined fusion plasma emits black body radiation over the frequency range in which intense RF heating has been demonstrated. The upper frequency limit may be set at the second harmonic of the electron cyclotron frequency(For example, see paper j5i2 by O. Motojima on p 1776, Bull. Am. Phys. Soc. Vol. 43, 8 (1998).). The lower frequency limit is set as zero. Assume an ideal fusion plasma of 10 exp 14 ions per cubic cm, an ion and electron temperature of 10 exp 8 degrees Kelvin, a magnetic field of 10 Tesla, and a volume of one cubic meter (Physicist's model.). Assume that this cube of plasma is emitting black body radiation from all six of its faces. Under these assumptions, the radiation lifetime for energy confinement is 11 seconds. This is rather close to the Lawson lifetime for break-even of one second. If emission occurs at higher harmonics, the situation is considerably worse.

[GP01.97] Advanced Tokamak Research at the DIII--D National Fusion Facility

The tokamak magnetic fusion concept has an enormous range of freedom to optimize its properties and potential as a fusion power system. Early tokamaks formed plasmas whose properties were largely derived from inductive pulse formation, with centrally peaked current profiles and low self-driven current fractions. In contrast, the AT concept utilizes: (1) non-monotomic current profiles, (2) electric and magnetic field shear to reduce plasma turbulence and subsequently energy transport; (3) plasma pressure and current profile control to optimize plasma pressure for high reactivity; (4) self-driven bootstrap currents to enable efficient steady-state operation; and (5) divertor design to provide power dispersal and particle control to sustain plasma purity with intense power and particle exhaust. This paper reports \hboxDIII--D experimental progress in these individual research areas as well as their optimized integration and theoretical modeling toward demonstrating the advanced tokamak concept.

[GP01.98] A Theoretical and Experimental Investigation into Energy Transport in High Temperature Tokamak Plasmas

In the design of next step fusion devices the energy confinement time (\tau_E) has been identified as a critical parameter. Efforts to predict \tau_E in future machines has centered around three different techniques: statistical analysis of global energy confinement data, a dimensionless physics parameter similarity method similar to aircraft wind tunnel experiments, and modeling of cross-field turbulence driven energy transport. Early statistical work proved quite accurate in predicting future machine performance. Valuable insight into energy transport has been obtained in wind-tunnel like experiments involving experimental data from the international fusion community. Recent advances in theoretical modeling have increased our understanding of the underlying physics that determines global confinement. New experiments using a modulated heat source and varying the ratio of electron to ion temperature have allowed detailed comparisons to theories. A discussion of the three techniques will be presented.

[GP01.99] A Comparison of Fueling with Deuterium Pellet Injection from Different Locations on the DIII-D Tokamak

Deuterium pellet injection has been used in experiments on the DIII-D tokamak to investigate several aspects of plasma confinement and density control. The measured fueling efficiency and deposition profiles from pellets injected from the outer midplane show a large discrepancy with pellet ablation theory, while the penetration depth compares favorably with theory. An apparent outward displacement of the deposited pellet mass is observed and is hypothesized to occur from \nablaB induced drift effects. Vertical injection of pellets 20 cm inside the magnetic axis has been employed on DIII-D to investigate these effects. The results show pellet mass deposition inside the expected deposition radius, suggesting that a drift of the pellet ablatant is occurring toward the low field side edge of the plasma. New injection lines have been installed on the inner wall enabling injection from the highest field region of the device. Details of the results from injection at these different locations including comparisons with the measured pellet ablation light emission and the theoretical ablation rate will be presented. Modeling of the drift of the pellet ablatant following the ablation process will be discussed.

[GP01.100] Effect of Trapped Electrons and Collisionality on Electron Cyclotron Current Drive and Comparison with Experiments on the DIII--D Tokamak

Experiments on DIII--D have shown that the normalized figure of merit n_eI_pR/PT_e for Electron Cyclotron Current Drive is independent of minor radius.(T.C. Luce et al.), to be published in Proc.\ IAEA Meeting, Yokohama, Japan, 1998. This is surprising since trapping of current-carrying electrons is expected to reduce the driven current for off-axis deposition. However, theoretical treatments of the trapping effect have been based on the collisionless assumption at all energies. This assumption is clearly invalid for lower energy electrons. We present a quantitative study of this effect by using adjoint techniques to calculate the collisionality dependence of the ECCD. Both approximate analytic and numerical solutions of the adjoint equation for current drive (without invoking bounce average) are considered. The impact of finite collisionality on off-axis ECCD on the \hboxDIII--D experiments and the projection to high performance advanced tokamak conditions in \hboxDIII-D will be discussed.

[GP01.101] Plasmas Near the Tokamak Divertor X--Point

The divertor \hboxX--point region, where four distinct plasma regions or quadrants meet, each with different temperature T, particle density n, pressure p=nkT, electric potential \Phi, and velocity v, is little studied. We measured the electron temperature and density, plus \Phi and v_\parallel in all four quadrants of diverted plasmas in the \hboxDIII--D tokamak for both standard and reversed toroidal magnetic field B_T directions. The data yield the first observations of a strong v_E=E\timesB/B^2 plasma flow in the divertor ``private'' quadrant and abnormally high potential and electron pressure, density, and temperature near the \hboxX--point. We compare the observations to \hbox2-D transport modeling by the UEDGE code, recently improved to run with all the classical transport drifts, which also shows strong v_E flow in the private quadrant. The v_E flow is found to be a major contributor to the observed B_T direction dependence of divertor properties, while the electron overpressures are produced by high ion temperatures upstream of the divertor.

[GP01.102] Enhanced Electron Cyclotron Current Drive in the DIII--D Tokamak Due to Localized Deposition

Off-axis electron cyclotron (EC) current drive (CD) efficiency in the \hboxDIII--D tokamak has been observed to exceed results calculated using axisymmetric Fokker-Planck (FP) theory. Preliminary calculations with bounce-averaged FP codes indicate the possibility of CD enhancement due to collisional reduction of trapping effects.(T.C. Luce et al.), IAEA Fusion Energy Mtg., Japan, 1998. We examine this further using the non-bounce-averaged CQLP FP code(O. Sauter, R. Harvey, and F. Hinton, Contrib.\ Pl.\ Phys.\ 34), 169 (1994). which solves for f(v_\parallel, v_\perp, distance-along-B), including the streaming operator. Electron current is driven by QL diffusion over a small region along B. To complete the circuit, current circulates along B for \sim100 toroidal turns, given by the poloidal (plasma circumference)\slash (beam size), a distance of order the mfp. Continuity is maintained by an effective electric field. The resulting modifications of the CD efficiency from bounce-averaged axisymmetric FP values, including collisional reduction of trapping effects, will be presented.

[GP01.103] Two Megawatt, 110 GHz Gyrotron for Plasma Heating

The gyrotron is under development as a high power source for plasma heating at electron cyclotron resonance. For heating large scale plasmas, such as the DIII-D machine at General Atomics, it is advantageous to have high unit power heating sources to reduce the cost and complexity of the system. We report on progress on research on a 2 MW gyrotron at a frequency of 110 GHz. The gyrotron is based on previous successful results at the 1 MW level at frequencies from 110 to 170 GHz. The baseline design is for a TE28,8 mode cavity with an electron beam of 80 to 110 kV and a current of up to 80A. The expected efficiency is at least 30% but it should increase to over 50% with a depressed collector. The output beam will be a Gaussian TEM00 beam in free space. The gyrotron will be investigated experimentally in short pulse operation (\sim3 microseconds) at MIT and, if successful, will be developed in a 10s pulsed or CW version by industry. High power gyrotrons are useful for many other applications including materials processing and advanced radar.

[GP01.104] Confinement and Heating of Deuterium-Tritium Plasmas in TFTR

The Tokamak Fusion Test Reactor was authorized for construction in 1976 with the mission to produce significant fusion power from a magnetically confined D-T plasma heated by beams of energetic neutral atoms. After its construction began, results from the smaller tokamaks then operating with NB heating suggested that the confinement and performance of TFTR would be less than originally projected. In physics experiments conducted in deuterium plasmas through the 1980s, regimes of improved confinement were discovered and developed to the point where projections of TFTR performance justified proceeding to D-T fuel in 1993. TFTR operated with D-T for over 3 years, achieving a peak fusion power of 10.7~MW, producing over 1~GJ of fusion energy and providing a demonstration of the feasibility of operating a fusion power plant. Detailed experiments were conducted to study the physics of the energetic alpha-particles produced by D-T fusion reactions and the confinement and heating of the thermal plasma in their presence. The progress both in plasma performance and in understanding transport and stability achieved during the life of TFTR suggest that fusion research is ready to advance to the study of burning plasmas, where the plasma heating by the alpha particles dominates the losses.

[GP01.105] Reducing turbulent-driven transport in fusion plasmas and identifying a common a common physics basis

In the last several years, plasmas with core ion thermal and particle transport that is at or below the fundamental limited predicted by collisional theory have been generated. These also exhibit dramatic reductions in turbulence thought to be responsible for normally larger transport rates. As these regimes join a longer history of transport bifurcations in the plasma edge, a scientific challenge is identifying a physics basis that unites these phenomena. To this end, a study of TFTR and DIII-D tokamak plasmas is presented. Results suggest that E \times B flow shear stabilization and decorrelation of turbulence accounts for many differences. To highlight common physics, the spontaneous generation of electric field shear layers in the core and edge is described. Also, similarity experiments on TFTR are outlined in which the radial electric field E_r was varied by modifying the momentum input. High transport and fluctuation levels returned when the E \times B shear was mimimized, but plasma dynamics, transport rates, and fluctuation levels similar to low transport DIII-D plasmas were observed when the DIII-D E_r structure was reproduced.

[GP01.106] Advances in Understanding of Magnetohydrodynamic Stability of Fusion Reactor Plasmas

The Tokamak Fusion Test Reactor produced plasmas with peak electron temperatures up to 13~keV, ion temperatures up to 45~keV and plasma pressures up to 7~atmospheres, parameters approaching those in a D-T fusion reactor. An extensive set of diagnostics was developed for the study of magnetohydrodynamic instabilities driven by plasma pressure, current and non-Maxwellian fast ion distributions, including the fusion alpha particles in D-T plasmas. These included coils for detecting external magnetic fluctuations from global instabilities and soft x-ray cameras, measurements of local electron temperature and microwave reflectometers for determining their internal structure. These diagnostics, complemented by comprehensive measurements of plasma equilibrium parameters, revealed a wide range of MHD activity. The discoveries included several types of kinetically driven global Alfvén instabilities, pressure driven global MHD modes (kinks and ``infernal'' modes) as well as pressure driven instabilities (``ballooning'' modes) localized by symmetry-breaking global modes, and a class of metastable tearing modes driven by kinetic (bootstrap) currents. Study of these phenomena stimulated great advances in MHD theory and in simulation codes in the last decade.

[GP01.107] Effects of the Hydrogenic Mass on the Energy and Particle Transport in JET and TFTR Discharges

Fusion experiments have been performed in JET and TFTR using mixtures of D and T isotopes. In TFTR, supershots exhibited a marked reduction of core energy transport with increasing fractions of T injection. In JET, ELMy H-modes exhibited relatively small changes, except in the edge pedestal region. An extensive campaign was recently conducted in JET using H, D, DT, and T to study this isotopic effect and the scaling in dimensionless and controlable parameters. The TRANSP plasma analysis code is used to analyse more than 60 discharges from each regime. A database is constructed of results during the quasi-steady state phases. Although the two regimes have very different plasma profiles and boundary shapes, they share some qualitative features. The \(T_i\) profiles are reproduced by a gyrofluid model of toroidal ITG modes, supressed by \(E_r\) shear. The supression is weak in the ELMy regime and strong in the supershot regime. Also, the central \(T_i\) exhibits an approximate power law scaling with the central hydrogenic isotopic mass A and the central beam fueling source rate. The A exponent is considerably larger for supershots. Both regimes exhibit a reduction of the ratio of the particle and energy transport coefficients in the core, with a similar power-law scaling with A.

[GP01.108] Simultaneous 3-D imaging of plasma density and temperature turbulence in toroidal plasma devices.

Transport physics based on turbulence in toroidal plasma device demands a decisive experimental measurement. In classical fluid dynamics, the physics understanding has been greatly stimulated by the visualization of turbulence via optical measurement. Advances in diagnostic concept and technology allow us to design a system capable of simultaneous 3-D imaging of the turbulence spectrum (density and temperature) in toroidal plasma devices such as tokamak and stellarator. This paper will focus on the details of the concept design such as accessibility, machine parameters, imaging system and relevant frequencies will be discussed for a various devices (e.g. LHD, TEXTOR, and DIII-D). Special attention will be given to obtain frequency and wave-number spectra with a sufficient spatial resolution so that the results can be readily compared with visual results produced by gyro-kinetic (GK) and/or gyro-fluid (GF) simulations

[GP01.109] Physics of Superthermal Ions in Tokamak Fusion Test Reactor Plasmas

In magnetically-confined fusion plasmas, superthermal ions with typical energies of tens to hundreds of keV are often used to heat the plasma to temperatures where the fusion reaction rate is significant, ie T_i>5~keV.

These ions can originate from the injection of energetic neutral atoms, the application of radiofrequency waves at the ion cyclotron frequencies, or as products of various fusion reactions.

For a tokamak plasma, in which the confinement of single particle trajectories is determined by fundamental symmetries of the configuration and conserved dynamical quantities,

several distinct categories of trajectories can be identified, which have different confinement properties.

Superthermal ions undergo collisions with the bulk plasma particles that can both affect their confinement and eventually cool the energetic ions while heating the plasma.

Collectively, a population of superthermal ions can act as a source of free energy to drive electromagnetic instabilities in the plasma.

Such instabilities can break the underlying symmetries and increase the rate of deconfinement of superthermal ions and bulk plasma.

Typical instabilities are the fishbone and toroidal Alfvén eigenmode.

Results from a variety of superthermal ion studies in the Tokamak Fusion Test Reactor will be presented.

[GP01.110] The Development of RF Heating of Magnetically Confined Deuterium-Tritium Plasmas

The experimental and theoretical development of ion cyclotron radiofrequency heating (ICRF) in toroidal magnetically-confined plasmas recently culminated with the demonstration of ICRF-heating of D-T plasmas, first in the Tokamak Fusion Test Reactor (TFTR) and then in the Joint European Torus (JET). Various heating schemes based on the cyclotron resonance between the plasma ions and the applied ICRF waves have been used, including second harmonic tritium, minority deuterium, minority helium-3, mode conversion at the D-T ion-ion hybrid layer, and ion Bernstein wave heating. Second harmonic tritium heating was first shown to be effective in a reactor-grade plasma in TFTR. D-minority heating on JET has led to the achievement of Q=0.22, the ratio of fusion power produced to power input, sustained over a few energy confinement times. In this paper, the key building blocks in the development of rf heating of plasmas will be reviewed and prospects for the development of advanced methods of plasma control based on the application of rf waves will be discussed.

[GP01.111] Fusion Ignition Research Experiment

The study of self-heated fusion plasmas is now the central physics issue for fusion research. The problem of coupling the requirements for adequate plasma confinement and magnetohydrodynamic stability with the unique non-linear characteristics a self-heated plasma is fundamental to the development of attractive magnetic fusion reactors. Recent advances in toroidal magnetic confinement experiments, theoretical understanding and modeling capabilities present new opportunities for investigating burning, magnetically confined plasmas at physical scales comparable to existing experimental facilities. Compact copper-conductor tokamak configurations with high magnetic field and operating at high plasma densities have advantages with respect to plasma confinement, beta limits, density limits, impurity control and fast alpha-particle confinement, and are well suited for studying burning plasma physics for the required time scales. The capability of a Fusion Ignition Research Experiment with R \sim 2~m, B \sim 10~T, I_p \sim 8~MA to produce, explore, optimize and develop fundamental understanding of strongly burning plasmas with a fusion gain Q \geq 10 for burn durations \geq 10 energy confinement times, based on existing tokamak science, will be described.

[GP01.112] Demonstrating the Feasibility of Self-sustained Fusion Burning Plasma Experiments

Developments in the understanding of the physics of high temperature, well confined plasmas have occurred in parallel to progress in the technology of the machines and of the relevant auxiliary systems (e.g. RF systems) that are needed to produce D-T plasmas capable of reaching ignition conditions. Thus a line of experiments with this objective has been proposed and the design of a specific machine (Ignitor) has been carried out(B. Coppi, et al., 17th IAEA Fusion Energy Conference \rm, (Japan) IAEA-F1-CN-69/FTP/14), supported by the construction of full size prototypes of all the key machine components. At ignition, the peak plasma pressure is in the range 1.5--4 MPa. At present, the requirements on macroscopic stability of the plasma column and on the confinement parameter n_0\tau_E where n_0 is the peak density and \tau_E the energy replacement time, lead to considering confinement configurations that feature both high plasma currents (typically I_p>10 MA) and high magnetic fields. Consequently the technology of cryogenic high field normal conducting magnets is the suitable one for near term experiments, while high field superconducting magnets technology that is relevant to foreseeable power producing reactors makes steady progress. Given the record high plasma densities that have been produced, with good confinement and purity quality, in high field machines, these lend themselves to be used for the exploration of fusion burn conditions of tritium poor plasmas (e.g. mainly D or D-He^3 plasmas) which require considerably higher n_0\tau_E than D-T burning plasmas and in which different physical processes play an important role.

[GP01.113] Reference Ignition Scenarios in Ignitor*

Progress in the understanding of the plasma transport, and an increased interest in compact D-T ignition experiments, have stimulated a renewed effort to investigate the expected performances of Ignitor and possible pathways to ignition. The degree of progress since the original work(B. Coppi, L.E. Sugiyama, M. Nassi,Physica Scripta \bf45)\rm, 112 (1992) was carried out has made it desirable to redo the reference scenarios. The initial current ramp plays a major role^1, for both plasma heating and stability, in an igniting plasma. Transport simulations carried out by means of the BALDUR(C.E. Singer, et al., \itComputer Phys. Comm. )49\rm, 275 (1989) code are used to establish new reference ignition scenarios, and their sensitivity with respect to transport models, plasma profile shapes and parameters during the current ramp. Ignition predictions using transport coefficients(B. Coppi, W. Daughton, Bull. Am. Phys. Soc. 43\rm, 1927 (1998) ) appropriate for the high density plasmas where T_e and T_i are comparable and the impurity level is relatively low are discussed.

*Supported in part by the USDoE.

[GP01.114] Fusion Energy Research on Alcator C-Mod

Alcator C-Mod is a high magnetic field (B_T = 8 T) compact (R = 0.67 m, a = 0.21 m) divertor tokamak that achieves high density (n_e \sim 1 - 4 x 10^20 m^-3) and high temperature (T_e,i \sim 4 - 5 keV) deuterium fusion plasmas. The magnetic geometry is elongated (\kappa \leq 1.8) with a D shaped cross-section. Plasma currents up to 1.5 MA have been produced. C-Mod has a closed divertor with molybdenum plasma facing components that withstand reactor-like parallel heat fluxes of \sim0.5 GW/m^2, yet maintains high plasma purity with Z_eff \sim 1.2. Ion Cyclotron Radio Frequency (ICRF) heating is applied in the range of 40 - 80 MHz with up to 8 MW of source power. A steady-state high energy confinement mode of operation is routinely achieved called Enhanced D_\alpha H-mode (EDA) that reaches up to twice L-mode confinement. The EDA H-mode is a reactor-relevant regime that combines high energy confinement with moderate particle confinement, which avoids impurity accumulation, and maintains steady-state without the high transient heat loads due to large edge MHD instabilities (ELMs) found in other tokamaks. Advanced tokamak operation through current profile control with Lower Hybrid Current Drive (LHCD) is also proposed on C-Mod.

[GP01.115] The physics of power dissipation in the Alcator C-Mod plasma fusion experiment

The power flow out of a magnetic fusion reactor core is expected to be \sim 1-2 GigaWatts. In the absence of volume dissipation mechansims, open magnetic field lines tend to create high power densities (\sim GW/m^2) flowing to material surfaces. However, a number of important physical processes can be used to distribute the power more uniformly over material surfaces including: the use of magnetic topology (divertor) to decrease the heat flux density; conversion of plasma power flow into radiation and neutral fluxes; and shaping of the divertor surfaces to spread the heat.

We present an overview of experiments in the Alcator C-Mod plasma fusion experiment related to issues of power dissipation and impurity control at the edge of hot (2-5 keV) plasmas. Up to 90% of the plasma power flow is dissipated under some operating conditions. This results in high density ( 1\times 10^21 m^-3), low temperature (\sim0.5 - 1.0 eV), and often recombining plasmas near material surfaces.

These plasmas exhibit a rich interaction of many fundamental physical processes, including plasma and impurity transport, atomic and molecular processes, and plasma-material interactions.

[GP01.116] Advanced Tokamak Mode of Operation in Alcator C-Mod

Alcator C-Mod is a shaped, diverted, compact, high magnetic field (8 T) tokamak operating at MIT. High performance can be achieved by operating at high inductive toroidal plasma currents (1.5-2.0 MA) for time durations of 1 s, or in the advanced tokamak (AT) mode of operation by reducing the magnetic field to 4-5 T, and extending the pulse length to 3-5 s, corresponding to several current diffusion times. The latter quantity is of considerable importance when we consider noninductive (RF) means to achieve the so-called reversed shear profile (non-monotonic q-profile) which has been found to be instrumental in other tokamak experiments to maintain good confinement by forming internal thermal barriers. In Alcator C-Mod an internal thermal barrier was achieved with pellet injection and ICRF heating (PEP mode), during toroidal current ramp (non-monotonic current profile). These modes do not last, however, possibly because of the transient nature of pellet injection. We have proposed adding off-axis current drive in the form of lower-hybrid waves, which should help in maintaining reversed shear equilibria, and hence extend the duration of internal thermal barrier. Experimental results on the PEP mode, and modelling of ERS equilibria for C-Mod will be presented.

[GP01.117] Wave-Particle Interactions in the Minority Ion Cyclotron Range of Frequencies (ICRF) in the Alcator C-Mod Tokamak

Wave-particle interactions between a compressional Alfvén wave and hydrogen (H) and ^3He ions have been investigated extensively in the Alcator C-Mod tokamak.(I.H.~Hutchinson et al., Phys. Plasmas \bf1), 1511 (1994) These ion species are a minority component in a deuterium (D) plasma. At toroidal magnetic fields of 4 - 8~T and a wave frequency of 80~MHz, the absorption mechanism is ion cyclotron damping. Heated minority ions collisionally transfer their energy to the majority plasma deuterons and electrons, resulting in efficient heating of the background plasma. As the minority ion concentration is increased the wave-particle interaction results in mode conversion of the compressional wave to an ion Bernstein wave with subsequent damping of the mode converted wave via collisionless electron Landau damping. Experimental measurements of minority ion cyclotron damping and mode conversion in C-Mod will be presented. The results will be compared with the predictions of 1-D and toroidal wave propagation, and absorption models and the limitations of these models will be discussed.

[GP01.118] Modelling issues in optically thick tokamak plasmas

In today's high-performance tokamaks, one can obtain plasmas opaque to the first few Lyman lines of deuterium, for example in recombining divertors or MARFEs. Opacity leads to nonlocal energy transport and strongly affects the ionization balance of the plasma. Since optical thickness is a sensitive function to details of the spectral line shapes, a self-consistent model of the line must be constructed. Stark and Doppler broadening, as well as Zeeman splitting, must all be included in the analysis. We report results from three different modelling efforts converging into one self-consistent model. We have used the CRAMD, CRETIN and IVKAN codes to compute effects of opacity and line shaping of, in particular, the Ly_\alpha, Ly_\beta, Ly_\gamma and D_\alpha lines of deuterium, for relevant plasma conditions. Results are compared with spectroscopic measurements from Alcator C-Mod. We present detailed benchmarks between the different approaches and report progress toward a unified approach including all the relevant physical effects.

[GP01.119] Formation of bound magnetic polarons in CdMnTe crystals

The present work reports the first experimental investigation of the contributions of the average exchange field and thermodynamic fluctuations of the magnetization to the magnetic polaron (MP) energy for the case of exciton-impurity complexes (EICs) in Cd MnTe crystals. The analysis of the experimental results was performed on the base of the analytic expressions [1] for spin splitting of the donor level by the MP effect using the effective values which introduced for the characteristic MP energy. The comparison of the value of the MP effect for EICs with that for the donor electron was carried out. The effect of an external magnetic field as well as doping of the experimental crystals with magnetic impurities (V and Co) on the manifestation of MP effects in low-temperature photoluminescence spectra was studied. The temperature dependence of the contributions of the average exchange field and thermodynamic fluctuations of the magnetization to the binding energy of EICs were determined. We are grateful to J.Spalek for a helpful discussion of the results obtained in this work. [1]. J.Spalek and J.Kossut, Solid State Commun. v.61, 483 (1987).

[GP01.120] Spin polarization of 3d transition metal ferromagnets probed by superconducting tunneling spectroscopy

Magnetic tunnel junctions (MTJs) are of great interest for their potential application as storage elements in magnetic random access memories. One of their advantages as compared to metallic giant magnetoresistive film structures is their larger relative change in resistance with field. In an MTJ the magnetoresistive effect is directly related to the spin polarization of the tunneling electrons of the tunnel current. The size of the effect depends on the relative ratio of majority to minority density of states (DOS), also called the polarization P (electrodes. We determine P using one superconducting electrode and one ferromagnetic electrode. Due to Zeeman spin splitting of the superconductor DOS, the relative difference in peak heights in a conductance curve is a measure for P in the ferromagnetic electrode. We recently observed polarization values in excess of 60AlCu/Al2O3/CoFe structures, the highest reported ever.

[GP01.121] Electronic and Magnetic Properties of Perovskite Oxides: LaMn_1-xCo_xO_3

Electronic structures of the doped perovskite oxides, LaMn_1-xCo_xO_3 (x=0.0, 0.25, 0.5, 0.75, 1.0), are studied using density-functional methods. In the 0.25 and 0.75 doped system, metallic property is found, while for the 0.5 doped compound with both orthorhombic and rhombohedral structure, the rare half-metallic nature is obtained. The density of states of lower Co doping compound has the similar features with the valence-band spectra obtained by experiment. The Co ions in the doped system have non-zero magnetic moments; they do not lie in the low-spin state, as in LaCoO_3 . The total moments of the doped compounds decrease with the concentration of Co dopant varying from 0.25 to 1.0, which are due to the decrease of Mn/Co ratio and the local moments of both Mn and Co ions in the doped compounds. The structural optimization has been done for the 0.5 doping compound. It is found that the orthorhombic structure with interlacing doping type is the most stable structure.

[GP01.122] NMR study of Mn doped yttrium orthoaluminates

Novel photorefractive materials, Mn:YAlO3 crystals, demonstrate permanent photocoloration and non-local photorefractive effect under laser light illumination. To characterize the crystals and better understand the nature of photoinduced effects, we study 27Al spectra, spin-lattice relaxation and transverse relaxation time T2 in YAlO3 crystals with different concentration of Mn ions, before and after exposure to laser light. The quadrupole coupling constant and principal axes directions were determined and compared with data for undoped YAlO3 crystals. Analysis of the kinetics of multiexponential spin-lattice relaxation and its dependence on temperature and Mn concentration allowed us to determine contributions of quadrupole interaction and paramagnetic impurity mechanisms. A strong increase of the relaxation rate after the exposure to laser light was observed. We discuss it in terms of photoinduced charge transfer between Mn ions and lattice defects, including possible role of polarons.

[GP01.123] Parity-locking effect of Anderson impurities on persistent currents in mesoscopic rings

The influence of Anderson impurities on the persistent current in a mesoscopic ring is studied using the Anderson Model, in which the s-d hybridization plays an interesting role on the phase dependence of the persistent current. In the weak coupling, the parity of the persistent current is locked when the Fermi-energy of the system is near the impurity level. In addition, the effect of the electron-electron interaction is also addressed under the Mean-Field Approximation.

[GP01.124] Moessbauer and X-ray Studies* of Chalcopyrite (CuFeS_2)

Moessbauer effect spectroscopy is used to study the magnetic and chemical properties of a natural crystal of chalcopyrite (CuFeS_2; from Western Mining Camps). X-ray diffraction data have also been recorded, indicating a minor trace of pyrite (FeS_2). Moessbauer parameters, among which magnetic hyperfine field, linewidth and isomer shift (IS), will be discussed and compared with literature values [1, 2]. Our main focus is covalency in CuFeS_2: the observed magnetic fields at the ^57Fe nuclei indicate a high-spin state of the Fe^3+ ions, whereas the relatively low IS points to a high degree of covalency. This mixed valency behavior is similar to that observed for the cuprate superconductors.

[1] H.N. Ok et al, Phys Rev B50 (1994) 10327. [2] C. Boekema, Ch 6, PhD thesis, University of Groningen (NL) 1977.

*) Supported by NSF grants DMR - 9706870 and PHY - 9605147, WiSE@SJSU and SJSU Graduate Studies.

[GP01.125] ESR and FMR studies in layered manganite: La1.2Sr1.8Mn2O7

Magnetization and ESR and FMR measurements at 9.4 GHz in a layered single crystal of La_1.2Sr_1.8Mn_2O_7 reveal magnetic ordering at 124(4) K and 280(2) K. Anisotropic FMR modes are observed below 350 K which we associate to the 280 K magnetic transition. Well above the transitions at 124 K the observed ESR at g = 2 has in general the same behavior than that found in the paramagnetic phase of the cubic perovskites, La_2/3R_1/3MnO_3+\delta (R = Ca, Sr, Pb). The dependence of the M and the resonance data with T and its association with 3D and 2D magnetic ordering will be discussed.

[GP01.126] Localization and Spin-fluctuation in Pr-based Manganites

--- A parallel study on the transport properties of Pr_0.7Sr_0.3-xCa_xMnO_3 and Pr_0.6Sr_0.4-yCa_yMnO_3 have been made to understand the mechanism of the colossal magnetoresistance (CMR). We have demonstrated the validity of the model of the polaronic localization for Pr_0.7Sr_0.3-xCa_xMnO_3. However, for the weak localization system Pr_0.6Sr_0.4-yCa_yMnO_3, we have found that the magnitude of CMR at the temperatures below the insulating-metallic transition temperature (T_p) is higher than that at T_p, and the value increases linearly with decreasing temperature. This result suggests that the spin-dependent localization above the magnetic ordering is not the essential mechanism for CMR. We have attributed the CMR effect in Pr_0.6Sr_0.4-yCa_yMnO_3 to the field-induced suppression of spin-fluctuations. * This work is supported in part by the National Science Council of the R. O. C. under grant No. NSC-88-2112-M-002-027.

[GP01.127] Magnetic Properties of Small Ferromagnetic Particles

The discovery of giant magnetoresistance (GMR) effect in inhomogeneous alloys of magnetic and non-magnetic metals has attracted a great deal of attention to these materials. Besides other properties, interest has been focused on magnetization curves of granular metals,from which information on the cluster size distribution can be obtained. Calculated curves give coercivity field strengths comparable to those of very small grain systems or fine powders, in good agreement with experiment. However, disagreement between theory and experiment exists on the saturation fields. Theoretical predictions are significantly smaller than the observed values. Thus, the discussion of the assumptions underlaying the models used in those calculations is clearly in order. By performing Montecarlo simulation on 2D systems consisting of N \sim 500 magnetic atoms, the magnetic ordering was investigated. We included anisotropy, exchange and dipolar interaction to couple the atoms. The result of the magnetization curve v/s temperature, when considering dipolar interaction among the atoms, show a significat lowering with respect to that considering only exchange and anisotropy interactions. In spite of the fact that dipolar interaction is very small compared with exchange, their role is to stabilize a domain structure over a broad temperature range below the Curie point.

[GP01.128] Electrical and thermodynamic properties of (La_1-xGd)_0.7Ca_0.3MnO_3

Electrical resistivity at ambient and high pressure (P below 7 kbar) and magnetization measurements were conducted on (La_1-xGd)_0.7Ca_0.3MnO_3 with x between 0 and 0.3. Canted ferromagnetic behavior was observed for x less than 0.2. The pressure dependencies of the magnetic transition temperatures T_c were found to be 14.6, 22.6, 23.0, and 16.4 K/GPa for x = 0, 0.1, 0.15, and 0.2, respectively. The results will be discussed within the framework of the double-exchange mechanism with strong electron-phonon interactions.

[GP01.129] EPR study of the many body S=1/2 chain end spins in the Haldane gap compound Y_2BaNiO_5

We have demonstrated the existence of the many body S=1/2 chain end spins in the linear chain compound Y_2BaNiO_5 using EPR to show the g-value dependence with orientation of external magnetic field and the temperature and concentration dependence of the signal intensity. We have compared the temperature dependence of the EPR signal with Mitra and Halperin's theory and the g-values with the single ion anisotropy determined from neutron scattering.

[GP01.130] Boundary conditions and thermodynamic limit for frustrated Ising lattices

Ising lattices with ferromagnetic (F) and antiferromagnetic (AF) interactions of equal strength present frustration. A plaquette (minimal closed circuit) is frustrated when it is formed by an odd number of AF interactions. It is usually assumed that the distribution of frustrated plaquettes defines the properties of the lattice in a unique way. In the present paper we present evidence showing that the way in which boundary conditions (BC) are applied to any plaquette distribution strongly influences the properties of the lattice. Combinatorial and topological techniques will be used to define different ways of applying cyclic BC to plaquette distributions in square lattices. Ground state properties are thoroughly calculated for each way of applying BC. Different results are obtained in each case; they will be discussed and compared to numerical results obtained for a large amount of randomly generated samples of the same size. We conclude that results can be strongly biased by the way cyclic BC are repeatedly applied to reach the thermodynamic limit.

[GP01.131] Effect of Roughness on Magneto-Optical Kerr Effect

It has been well established that ab initio calculations of the magneto-optical Kerr effect (MOKE) agree well with experimental results for 3d-transition metals, multilayers and compounds. To date, such calculations have been confined to bulk compounds or multilayers with smooth interfaces. It is recognized that inter- facial roughness plays an important role in magneto-resistance and interlayer coupling in magnetic/non-magnetic sandwiches. We now investigate how roughening of the interface affects the MOKE of Fe/Pt multilayers. We have performed MOKE calculations on several Fe/Pt multilayers with a Full-Potential Linear Muffin-Tin Orbital (FPLMTO) electronic structure code and obtained good agreement with experiment for Fe/Pt in the L1_0 structure. To study the effects of interfacial roughness, we now use a Tight-Binding Linear Muffin-Tin Orbital with Atomic Sphere Appromixation (TB-LMTO-ASA) electronic structure code where we can reasonably study about two hundred atoms in the unit cell. First, a comparison of the accuracy of the TB-LMTO-ASA to that of the FPLMTO will be made for the Fe/Pt L1_0 structure. Then a study will be made on a much larger unit cell with the latter method so that interdiffusion at the interface can be taken into account and its effect on the magnetic moments and MOKE can be examined.

[GP01.132] Low-Field Giant Microwave Magnetoimpedance in CMR Manganite Powders

About one year ago(V.V. Srinivasu, et al.), J. Appl. Phys.83, 2866 (1998). we discovered that in micron-size powders of the CMR manganites La_0.7Sr_0.3MnO_3 and La_0.7Ba_0.3MnO_3, there is a sharp increase in the zero-field microwave absorption when the temperature T drops below T_C. Further, it was found that this rise could be largely suppressed by application of a dc magnetic field of a few hundred Oe, parallel to the microwave magnetic field. The measurements have now been extended to several other manganites and we will present unequivocal evidence that both the above phenomena involve microwave absorption in the spin system. That is, the total loss P can be written as P = P_CE + P_spin where the former represents the contribution due to conduction electrons and is therefore dominant only at T > T_C and only weakly dependent on the magnetic field. P_spin on the other hand, increases rapidly for T < T_C and is strongly influenced by a magnetic field, hence a giant magnetoimpedance. It will be shown that P_spin follows the empirical expression P_spin(H)/P_spin(0) = H_0^2/(H^2 + H_0^2) where H_0 is a temperature dependent material parameter. Supported in part by NSF MRSEC Grant No. DMR96-32521.

[GP01.133] Giant Magnetoresistance and Interface Roughness: A Quantitative Study under Elimination of Bulk Defects

The Giant Magnetoresistance (GMR) observed in magnetic multilayers is due to spin-dependent scattering of electrons at interfaces or bulk defects. Studies on polycrystalline samples with their inherent high bulk defect density do not allow to separate those contributions. We produced Fe/Cr superlattices without bulk defects, thus providing only interface scattering. The vertical and lateral interface roughness parameters were quantitatively analyzed using X-ray diffraction and Conversion Electron Mößbauer Spectroscopy. The magnitude of the GMR effect increases with increasing roughness amplitude and decreasing lateral correlation length of the roughness.

[GP01.134] Amplification of Magnon Sound by a Constant Electric Field in Ferromagnetic Semiconductors

The nonequilibrium kinetics of the electron-magnon-phonon system of a ferromagnetic semiconductor is studied. It is shown that mutual electron-magnon dragging and heating take place in the system (phonons play the role of a thermal bath). This radically changes the energy distribution of the electrons in the strong electric field and may lead, e.g., to the negative differential conductivity. It is found that there are quite favorable conditions for a propagation of the magnon-temperature or magnetization waves, which we call the magnon sound. The magnon sound can induce an alternating magnetic field, and hence both magnetic disturbances and electromagnetic radiation may be experimentally observed. We predict the amplification of the magnon sound by the electrons drifting in a constant electric field E. The amplification ensues when the drift velocity of the magnon gas exceeds some threshold value which has the order of a magnon sound velocity V=a(T/M)^1/2, where T is the phonon temperature, M is the magnon "mass" and a is the order of unity. Waves of magnon temperature and drift velocity propagate along the direction of E. Growth of amplitudes of these waves one may consider as an instability in the magnon gas moving with the "super-magnon-sonic" speed.

[GP01.135] Structural and magnetic ordering effects in the Raman spectra of LaMnO_3+\delta

Temperature dependent Raman spectra of polycrystalline LaMnO_3+\delta samples were correlated to their structural and magnetic properties for different oxygen contents. For antiferromagnetic orthorhombic samples \delta =0.00 and \delta =0.03) a softening of the 610 cm^-1 Raman mode is observed below T_N. This is interpreted in terms of a spin-phonon coupling caused by a phonon modulation of the superexchange integral. The rhombohedral sample (\delta =0.14) presents two Raman modes at \sim 490 and \sim 620 cm^-1 which are not allowed for the D_3d^6 symmetry. Indeed, x-ray measurements in this sample show Bragg peaks of D_2h^16 space group, indicating the presence of orthorhombic regions.

[GP01.136] Phonon Raman scattering in pyrochlores A_2Mn_2O_7 (A = Tl, In, Y)

Raman scattering of optical phonons in polycrystalline Tl_2Mn_2O_7, In_2Mn_2O_7 and Y_2Mn_2O_7 has been measured as function of temperature. In the Tl and In compounds we found an increase of the \sim 280 cm^-1 phonon frequency below the magnetic ordering temperature. We show that the deviation of this phonon frequency from the Grunneisen's law below the transition temperature scales with the square of the measured magnetization. This phonon renormalization is interpreted in terms of a spin-phonon coupling due to a phonon modulation of the Mn-O(1)-Mn superexchange integral. Our results suggest that the superexchange interaction play an important role in the mechanism of the magnetic ordering.

[GP01.137] Large magneto-optic response from magnetic semiconducting layered structures

Theoretical analysis shows that (magnetic semiconductor)/metal layered structures, or thin films, with properly matched parameters can be very effective in selecting states of a certain helicity for reflected or transmitted light. Numerical calculations carried out for EuS/Ag structures indicate that for reflected light with an energy near the band gap edge, the Kerr rotation can be as large as 90 degrees, with very large reflectivities. An applied magnetic field can be used to change the helicity selection. A number of examples will be presented.

[GP01.138] Interface Roughness studies for GMR using GIXR

We have used Grazing Incidence X-ray Reflectometry (GIXR) to study the influence of structure parameters, v.g. interface roughness, on spin valves exhibiting Giant Magneto Resistance (GMR). The experimental results are modelled and the extracted parameters are presented and compared.

[GP01.139] Electron-Doped Manganese Perovskites: The Magnetic Polaron State

Using the Lanczos method in linear chains we study the ground state of the double exchange model including an antiferromagnetic super-exchange in the low concentration limit. We find that this ground state is always inhomogeneous, containig ferromagnetic polarons. The extention of the polaron spin distortion, the dispersion relation and their trapping by impurities, are studied for diferent values of the super exchange interaction and magnetic field. We also find repulsive polaron polaron interaction.

[GP01.140] High dielectric permittivity and magnetoresistance of (La_0.7Ba_0.3)(Mn,Co)O_3 system

Eleven samples of (La_0.7Sr_0.3)(Mn_1-xCo_x)O_3 CMR system with x = 0 to 1.0 have been well prepared using solid state reaction method. The single phase of pseudeocubic with lattice constant of ~ 3.98 A was observed in these samples by X-ray diffraction patterns. The measurements of the complex dielectric permittivity of those samples were carried out as a function of Co doping (x = 0 - 1.0), frequency (20 Hz- 10 MHz), and tempearture (4 K - 300K). The results show strong temperature and frequency dependence. The effect of the cobalt doping on magntoresistance and magnetic properties have also been studied in this report.

[GP01.141] Magnetic Light Scattering on Metal-Oxide Antiferromagnets

In Antiferromagnets a rather strong response in inelastic light scattering can be observed due to a two magnon excitation. HTc-cuprates and NiO have been studdied in comparison because both exhibit high energy magnons at the zone boundary. We have performed two Magnon Raman scattering over a wide Temperature range with several laser excitations on both systems and Brillouin scattering of one magnons in NiO. The interpretation of magnetic Raman scattering in the HTc-cuprates is controversal. In our measurements we find rather good agreement with 3-band Hubbard calculations which do not only explain the rough excitation profile but also the absolute scattering efficiences. The Raman vertex scales with the exchange J wich is experimentally verified with the absolute scattering rate of NiO. The interpretation of the 2-Magnon lineshape of the HTc-cuprates requires additional Magnon-Phonon-interaction. In contrast, NiO does not show strong Magnon-Phonon contributions but also does not behave exactly as a simple antiferromagnet in spinwave approximation. The Brillouin signal of one-magnon excitations in NiO has been measured depending on wavevector direction. Our results do not support the earlier interpretations with surface magnons.

[GP01.142] ESR of a Radiation-Induced Defect in Paratellurite

Gamma, uv or electron irradiation of undoped \alpha-TeO_2 single crystals at 77 K produces a new electron-like defect. The defect was observed in crystals that were previously electron irradiated at 400 K. The ESR spectrum is observable only at low temperatures and reveals a single spin-1/2 defect in eight inequivalent sites with principal g values of 1.8181\pm 0.0005, 1.8059\pm 0.0005, and 1.8620\pm 0.0005. Resolved hyperfine structure is attributed to magnetic isotopes of tellurium in several different sites. At low temperatures this center can be bleached with 600 nm light and reproduced with 365 nm. This new defect has many characteristics similar to those of the dominant intrinsic defect in electron irradiated TeO_2, which is an oxygen vacancy that has a net charge +e with respect to the perfect crystal. A possible model for this new defect is an oxygen vacancy center having a net charge -e with respect to the perfect lattice.

[GP01.143] Effects of loss in piezoelectric ceramic resonators

Piezoelectric resonators and filters are important components in many military and commercial communication systems. Standard piezoelectric materials such as quartz have low coupling values and are quite expensive to fabricate. Newer materials like piezoceramics have been developed that are cheaper, have higher values of coupling constant but can be considerably more lossy than single-crystal materials. The manufacture of these ferroelectric ceramics often leads to unacceptable process deviations from batch to batch. By gaining a better understanding of the material loss mechanisms involved, and equivalent networks to characterize resonators made of these materials, it should be possible to improve the situation in two regards. First, the circuit values can be accurately interpreted in terms of the physical properties to provide better feedback to the factory. Second, the circuit parameters of the ceramic can be ``impedance-matched'' to the using circuit in a manner which will make the best use of the acoustic device. The purpose of this work is to explore loss mechanisms through the use of complex elastic coefficients in the piezoelectric constitutive relations and the differential equations for a simple one-dimensional resonator. Results will be presented that show how the complex elastic coefficients modify the acoustic wave velocity, the coupling constant and the resonant frequencies of the resonator. An equivalent circuit model of a resonator will also be presented with and without loss to compare with the calculated results.

[GP01.144] ^19F^\ast Nuclear Quadrupole Interaction in Solid Hydrogen Fluoride

We have studied the electronic structure of solid hydrogen fluoride by the Hartree-Fock Cluster procedure. For simulating the infinite solid, chains of HF molecules in the solid ranging in length from 2 molecules to 21 molecules are studied as well as clusters involving interactions between adjacent chains. The results allow one to identify the source of the observed quadrupole coupling constants e^2Qq of 34\pm 1MHz and 40MHz for the excited state ^19F^\ast of the fluorine nucleus obtained from Time Differential Perturbed Angular Distribution(TDPAD) measurements(E.Bertholdt et al, Hyperfine Interaction \underline34), 193(1987). The former e^2Qq is assigned to the bulk solid while the latter is assigned to the small fragments of two or three molecule units produced by the experimental procedure for implanting the excited ^19F^\ast nucleus.

[GP01.145] Appearance and Disappearance of Free Ion Pairs In NaCl Crystals and the Diredt Result of the Anderson Experiment.

When an 8 eV quantum of UV radiation is absobed in an NaCl crystal, a free (slightly mobile) Na^+ Cl^- ion pair appears in the crystal. After a certain lifetime, the pair disappears with an emission of energy, totaling 8 eV. To one who does not know (or does not want to know) that the ions were there before the absorption and remained there after the emission, this means, by Einstein's resume,("The mass of a body is a measure of its energy content; if the energy changes by E, the mass changes in the same sense by E/c^2)." that 8 eV is equivalent to, or creates, 58 amu, and vice vesa. In 1932, C.D.Anderson proved that a free electron positron pair appears in any point of space when1.02 MeV is absorbed there, and that the pair disappears with the emission of quanta totaling 1.02 MeV. Without bothering to find out, as we did,(M.Simhony, The Epola Space, 1990, 160 pp, and The Story of Matter and Space, 1998 70 pp (available from the author). M.Simhony, Invitation to the Natural Physics of Matter, Space, and Radiation, World Scientific, 1994 (292 pp).) that the particles were there before absorption and remained after the emission, it was canonized that thepair is created by 1.02 MeV and annihilates into this energy, though with up to a million times higher energy "bombs", developed since, nobody has ever "created" of destroyed a SINGLE electron or positron.

[GP01.146] The study of sublayer spacing and electronic structure for MxTiS2(M=V,Cr,Mn,Fe, Co,Ni)

The total energy curves of intercalation compounds MTiS2 (M=V,Cr,Mn,Fe,Co,Ni) and Fe1/3TiS2 as a function of S-M-S sublayer spacing c1 which corresponds to the atomic positions of M are calculated by self-consistent LMTO-ASA method. The optimized c1 values are then obtained. In carrying out the optimization, the Ewald correction to the LMTO-ASA total energy takes an important role. The optimization of lattice constants is also carried out. Using the optimized atomic positions and structural parameters, the relevant band structure, partial l-decomposed density of states, and bonding charge density(calculated by LAPW) are systematically studied. The dependence of these features and their trends on M, x and spin states are discussed and compared with some experimental results [1] (eg. electronic specific heat coefficient and transport properties). This work is supported by the National Natural Science Foundation of China.

[1]M.Inoue,H.P.Hughesi, A.D.Yoffe:Adv.Phys.38(1989)565 and references cited therein.

[GP01.147] ^11B NMR Studies in Li_2B_4O_7, LiB_3O_5 and BaB_2O_4 Single Crsytals

By using NMR method, the nuclear quadrupole interactions of the ^11B nucleus in Li_2B_4O_7, LiB_3O_5 and \beta-BaB_2O_4 single crystals have been investigated. From the angular dependences of the ^11B NMR in three mutually perpendicular planes measured at room temperature, we determined the quadrupole coupling constant (qcc), the asymmetry parameter (\eta) and the electric field gradient tensors. The qcc ranges from 2.4 to 2.7 MHz and the \eta is from 0.16 to 0.7 for 3-coordnated borons, whereas those of 4-coordinated ones are 177 \sim 527 kHz and about 0.6, respectively. These parameters appear to be new results. Furthermore, we found an interesting correlation between the asymmetry parameter of the 3-coordinated boron nucleus and the non-linear optical coefficient of these crystals. Thus, the ^11B NMR study of inorganic optical borate compounds may be a new microscopic technique for their non-linear optical properties.

[GP01.148] Overtone piezoelectric resonances in ceramic perovskites

Atomic theories of piezoelectricity can hardly be said to have passed a preliminary and tentative stage. In this work piezoelectric resonances (both thickness and radial) have been experimentally investigated in thin ceramic disks of Zr-rich PZT. The simple microscopic approach presented here, not an ab-initio calculation, describes quite well the observed dielectric response around the resonances, allowing the quantitative prediction of the piezoelectric moduli (d_33, d_31) and the electromechanical coupling constants (k_t, k_p) in terms of microscopic quantities. The agreement with the observed data is good for the theoretical expressions [\epsilon '(ømega), \epsilon "(ømega)] in a wide frequency range. An analysis of the characteristic parameters describing successive thickness and radial overtone is supported by the experimental data.

[GP01.149] Raman Study of Gel-Derived Al_2O_3 Doped with Eu^3+

The sol-gel technique is an excellent method to process ceramics including Al_2O_3 [1]. Here we show that when Eu is used as a dopant in this method, it affects the crystallization behavior. Our samples were prepared using alumina oxidation gel according to Ref. [2], doped with Eu (which enters the material as Eu^3+) at concentrations of 7 percent and heated to 850^0C or 1200^0C. We measured the sample's Raman spectra as excited by the Ar^+ laser line at 488 nm. Mariotto et al [1] found that alumina gel doped with Na remains amorphous at temperatures up to 1000^0C. Our Eu-doped gel, however, displayed the Raman peaks characteristic of crystalline structure after heating only to 850^0C, showing that doping with Eu decreases the crystallization temperature (T_C) compared to Na doping. We suggest that the decrease in T_C is due to the large ionic radius of Eu^3+ compared to Na^+, making the system more stable at high temperature. The decrease in T_C is significant in extending the use of the sol-gel technique to make alumina ceramics for applications. [1] G. Mariotto, et al., J. Solid State Chem. 86, 263 (1990). [2] G. Chen et al., J. Mater. Sci. Lett. 14, 1707 (1995).

[GP01.150] Biofilm - Surface Interactions Using X-ray Photoelectron Spectroscopy

Virtually all surfaces immersed in water for any period of time are colonized by microorganisms. These organisms adhere to the surface by producing extracellular polymers, predominantly polysaccharides. Biofilm formation and resulting biofouling cause serious problems for heat transfer equipment because they inhibit water flow and degrade the heat transfer coefficient. Conventional coatings cannot be applied to materials used for this application due to degradation of the heat transfer coefficient. Titanium, which is often the material of choice for heat transfer applications because of its corrosion resistance, is very prone to biofouling. This paper will report on the use of x-ray photoelectron spectroscopy to examine the interfacial chemistry between biofilm components such as alginic acid and gum arabic and a titanium surface. XPS is used to quantify the adsorbate bound to the surface under various conditions (including pH and salt content of the water), and to evaluate the adsorbate-surface bonding mechanism. Information regarding the biofilm-surface chemical interaction will be useful in developing better fouling resistant surfaces.

[GP01.151] Low Field MRI of Laser Polarized Noble Gas

We describe a device for magnetic resonance imaging (MRI) of laser polarized noble gas at low magnetic fields (below 100 G). The system is robust, portable, and inexpensive, and provides gas-phase imaging resolution comparable to that of high field commercial instruments (which operate at fields \sim 1 T). At 21 G, we have imaged laser polarized ^3He gas in both sealed glass cells and excised rat lungs. We have also demonstrated the utility of low field noble gas MRI in: (i) reducing magnetic susceptibility effects; and (ii) imaging voids within conductors. We describe practical considerations for working at low magnetic fields, and present measurements of radiation damping of laser polarized ^3He in this system.

[GP01.152] A Magnetic Resonance Study of Water Diffusion in the White Matter of the Brain

Magnetic Resonance (MR) Studies of water diffusion in white matter have shown that the directionality of the axonal system has an important effect on the apparent diffusion coefficient D_a. The axon fibres act as restricting structures on the diffusional motion of water molecules, causing a variation of D_a as a function of diffusion time T_d and direction of measurement. The Pulsed Gradient Spin Echo (PGSE) technique, Stimulated Echo (STEAM) technique and Alternated Gradient Pulse Sequences (AGPS) have been used to measure D_a for water in human white matter. D_a was measured over a large range of T_d (14-1000 ms) for diffusion parallel to the fibre direction and diffusion perpendicular to the fibres. The same experiments have been carried out on a phantom modelling the white matter axons. Phantom results were compared to D_a values obtained from in vivo MR measurements. Theoretical expressions and Monte-Carlo simulations describing the spin diffusion in cylindrical geometry have also been used to validate phantom D_a values.

[GP01.153] A Unique Optical Arrangement for the Rapid Acquisition of Spectrally Resolved Confocal Images

A confocal scanning microscope with a unique optical configuration for rapid acquisition of spectrally resolved images is described. The novel aspect of the optical configuration is the location of the detection device, which is placed in an intermediate position between fully descanned detection and non-descanned detection (frequently used in two-photon confocal imaging). This placement allows for the practical implementation of spectrally resolved confocal imaging. An immediate application for this technology is the supplementation of information gained in a standard confocal image, but it also offers the possibility of an extreme improvement to the detection means in modern methods of genetic sequencing.

[GP01.154] Do butterfly thin films act as solar collectors?

Many butterflies and moths display highly saturated iridescent colors which are due to thin films on the surface of some of the scales covering their wings or bodies. Miaoulis and Heilman (Ann. Ent. Soc. Amer. 91(1):122-7 (1998)) have claimed that the alternating submicron thick layers of air and chitin which produce these colors may serve as solar collectors which aid the insect's thermoregulation. This claim is made largely from analogy with their own observations of anomalous heating in nanopatterned semiconductors, and from specular reflectance data from the scales. We have completed a more thorough analysis of absorption in the thin film structures, and report our own measurements of the optical properties and consequent energy budgets of various parts of the wing structure of the iridescent Morpho menelaus, comparing its energy budget both with and without the thin films which cause its striking blue color. These measurements cast doubt on any significant thermoregulatory role for the films.

[GP01.155] Synthesis of Peptides and Oligonucleotides on Clay Mineral Surfaces: A Scanning Force Microscope Study

The technique of scanning force microscopy is used to investigate the prebiotic condensation reactions of amino acids and activated nucleotides on the surface of the clay mineral hectorite. Small oligomers of glycine are observed to form at step edges and micro-pore sites on the clay surface. Oligomers containing adenine are observed to form, but at more numerous surface locations. The role of the clay surface structure and exchangeable metal cations in these simulated prebiotic reactions will also be discussed.

[GP01.156] Enhanced Mass Sensitive Acoustic Wave Microsensors

Acoustic Plate Mode (APM) and Shear-Horizontal Surface Acoustic Wave (SH-SAW) devices have been used in liquid environments, which qualify them to be used for many bio, physical, and chemical sensing. For most bio-sensor applications sub-monolayers of bio species ( eg. mass density of a close-packed monolayer of IgG POD-conjugate-used as antigen- is about 2.5 ng/mm^2) needs to be detected. In the present work, mass sensitivity and the minimum mass detection limit is improved in terms of enhanced stability of the measurement system, and higher frequency of operation. The pulse ultrasonic measurement system incorporates a boxcar and an integrator which are used to suppress low-frequency base-line drifts and high-frequency noise. Higher frequencies of operation (400 MHz and up) devices are fabricated with high-resolution e-beam lithography.

[GP01.157] Confocal Microscopy without a Confocal Microscope--Using DIC Imaging to Reconstruct 3-D Images

Traditionally the 3-D reconstruction of microscopic objects has required the sectioning capability of a confocal microscope and the use of fluorescently-stained specimens. We present a simple computational technique for transforming multiple shadow-cast DIC (Differential Interference Contrast) micrographs into 3-D cube images. The contrast generation is sensitive to small refractive index mismatches in the specimen. It was possible to reconstruct the 3-D structure of nearly index-matched colloidal gels and crystals, up to several dozen layers deep. The same instrument was able to visualize sub-micron organelles and membranes in human endothelial cells, all without fluorescence. This technique is far cheaper than conventional confocal laser-scanning microscopes (CLSM), and acquires the data at a higher speed.

[GP01.158] MRI Electromagnetic Field Intensities And Local Power Deposition In Spherically Layered Configurations

The Extended Boundary Conditions formalism was used for the calculation of internal local electric field intensities and power deposition rates in human tissue for MRI-imaging configurations. The case of brain geometry has been studied in detail for 60MHz and 170MHz exciting EM fields. Of main importance is the study of local power deposition at the boundaries between different tissues. A detailed analysis is presented about how the field intensity is distributed and absorbed along the spherically layered geometry and the boundaries as well as the different features corresponding to the increasing frequency specifications of modern MRI systems.

[GP01.159] Optimization of Gradient-Echo Imaging for Hyperpolarized ^129Xe Gas

Laser-polarization of ^129Xe enhances its nuclear polarization by up to five orders of magnitude. This huge signal available can be used as a powerful probe for Magnetic Resonance (MR) imaging. The recent availability of large amounts of hyperpolarized ^129Xe gas may allow novel MR imaging techniques such as imaging of gas spaces like the lungs or blood flow measurements.

Due to the non-renewable nature of the hyperpolarized xenon magnetization, compared to conventional proton magnetization, special imaging considerations are required. This study explores the optimization of MR imaging of hyperpolarized ^129Xe gas with Gradient-Echo (GE) imaging in 2D and 3D. Constant and Variable Flip Angle techniques (CFA vs VFA) are demonstrated and compared, as well as sequential and centric phase-encode ordering. Finally, \itin vivo imaging and spectroscopy are demonstrated.

[GP01.160] Dynamics of Temporal Learning Rules in Adaptive Biological Networks

The exact timing of pre- and postsynaptic events has been shown experimentally to determine the magnitude of long-term changes of synaptic efficacy in biological neural networks. However, this temporal dependency of synaptic learning rules varies between different brain structures and different organisms. An analytic method is presented here for studying the dynamics of adaptive change given a temporal learning rule. The neural response dynamics that result from various learning rules are analyzed for a stochastic neuron that receives a temporal series of inputs via adaptive synapses. The temporal learning rules are constructed from associative enhancement/depression components in addition to a non-associative component that depends only on presynaptic activity. For a small region in the parameter space that describes the learning rules, the adaptive inputs generate a temporal pattern that exactly cancels any additional, non-adaptive input to the neuron. Other temporal learning rules lead to oscillatory instabilities and travelling waves that suggest a revision in our understanding of biological learning. Examples are given of biological systems were these temporal learning rules have been identified, and functional consequences discussed.

[GP01.161] Molar Model of Spatial Memory: Accuracy Data and Reaction Times

We have previously reported(P. Allen, M. Kaufman, A. F. Smith, R. E. Popper, Psychology and Aging 13, 501 (1998) and Experimental Aging Research, 24, 307 (1998).) on a statistical thermodynamics model of spatial memory. Unlike other applications of statistical physics to cognitive science our model is macroscopic. We use this molar neural model to directly analyze experimental data on one-dimensional and two-dimensional spatial memory tasks. We are in the process of incorporating in the model reaction times along with the accuracy information. The main assumption is that the reaction time is determined by the complexity of the task as measured by the entropy. We plan to analyze the available experimental data on both reaction times and accuracy data. This work is supported by NIH/NIA grant AG09282-06.

[GP01.162] Computational Modeling of Orientation Tuning Dynamics in Monkey Primary Visual Cortex

In the primate visual pathway, orientation tuning of neurons is first observed in the primary visual cortex. The LGN cells that provide the input to V1 are not orientation tuned, but some V1 neurons are quite selective. Two classes of theoretical models have been offered to explain this selectivity: feedforward models, in which inputs from spatially aligned LGN cells are summed together by one cortical neuron; and feedback models, in which an initial weak bias due to convergent LGN input is sharpened by intracortical feedback. Recent data on the dynamics of orientation tuning, obtained by a cross-correlation technique, may help to distinguish between these classes of models. To test this, we executed the reverse correlation protocol on various models, including a simple feedforward model: a spatio-temporal filter followed by an integrate-and-fire spike generator. The computational study reveals that simple feedforward models may account for some aspects of the experimental data, but fail to explain many salient features of orientation tuning dynamics in V1 cells. A simple feedback model of interacting cells is also considered. This model is successful in explaining the appearance of Mexican-hat orientation profiles, while other features of the data remain unexplained.

[GP01.163] Synaptic Unreliability Enhances Information Transfer by P-Unit Electroreceptors

We consider the effect of synaptic unreliability on signal encoding by sensory afferents of the weakly electric fish A. Leptorhynchus. This fish continuously emits a quasi-sinusoidal electric organ discharge (EOD). It probes its environment by detecting modulations of the EOD amplitude with P-type tuberous receptors. Each of these "probability coders" has its own baseline firing probability "P" per EOD cycle, determined by synaptic variability and other receptor noises; P also increases with EOD amplitude. The transfer of information about physiologically relevant random EOD modulations to output spike trains was quantified using the stimulus reconstruction technique. Spike trains were generated by biophysically plausible models of these receptors driven by random EOD modulations. We find that information transfer depends on the P-value, and is maximal for P less than one. Our results point to an essential role for synaptic unreliability in this sensory system.

[GP01.164] Novel method for in-vivo detection of tissue response to radiation

This work is targeting cancer treatment by radiation therapy. We propose a new and innovative method, in-vivo and non-invasive, for tumor and healthy tissues control during and after radiation treatment. Our preliminary data indicates that \alpha and \beta dielectric dispersions can be related to free-radical dynamics during irradiation, to changes in both extracellular fluid and cell membranes state after irradiation. \alpha dispersion is mainly due to extracellular fluid but tissue specific effects that might also contribute are counterion diffusion, membrane conductance and the charging of intracellular membrane bound organelles. \beta dispersion occurs at radiofrequencies, arising principally from the capacitive charging of cellular membranes in tissues. The long term potential of this study is directly related to the possibility of measurement of changes in tissue dielectric properties during treatment to quantify and predict individual responses to radiation.

[GP01.165] Ab initio study of the open state in DNA

Several modeled theoretical works have been carried out for the open states caused by local deformations in DNA. These models are based on the Dynamic Plane Base Rotator (DPBR) approach, but simplify the potentials of the base-stacking as well as the interbase H bondings. In the present work, we exactly introduce the realistic potentials of the above interactions by ab initio calculations, and investigate the generation and propagation of the open states in DNA double helices. The actual potentials reveal that the open state does not propagate as a sine-Gordon type solitary wave but rather play a stationary wave. This is because the actual potential strongly differs from the conventional cosine potential which produces the sine-Gordon solitons. An inclusion of the sugar phosphate backbone strengthens the stacking potential and causes a possibility of another type of the DNA open states.

[GP01.166] Recipes for Biomaterials in MRI

In recent MRI (Magnetic Resonance Imaging) literature it has been recognized that the algorithms for image reconstruction of organs or tumors need to be further improved in order to render reliable images. Outmust interest is the reduction of size as a function of time of a tumor while been treated with chemotherapy. To provide a guidance towards the proper algorithm it is necessary to check the reconstructed image against the real one. Of course this cannot be done with patients so one needs to rely on phantoms. In order for phantoms to be useful this way they need to be made up of well characterized materials. It is desirable to build samples that mimic the human body both geometrically and dielectrically. Although work is still been carried out measuring dielectric properties of human tissue there is already there is already a vast amount of information in the literature. In this work we will construct gelatins that have the same values of conductivity and permittivity as human tissue. This is achieved by dissolving various kinds of salts and acids in a variety of concentrations before allowing for the gelatin to crosslink. This work has been supported by a grant from NSF-DMR-9872689.

[GP01.167] Modification of LDR irradiation by Camptothecin: an inhibitor of DNA topoisomerase I

In brachytherapy, which uses the principles of Low Dose Rate (LDR) irradiation, the dose is centred on the tumour tissue. This means that the use of a radiosensitizer selective to tumour cells would, theoretically, lead to greater therapeutic gains clinically than in teletherapy. Camptothecin (CPT), currently in use as a chemotherapeutic drug, acts by the inhibition of Sublethal Damage Repair (SLDR). It does so by preventing the action of the topoisomerase I (topo I) enzyme. Using a 1 micromolar concentration of CPT, we have applied the LDR and CPT treatments concomitantly. Cell survival is determined by clonogenic assay. The number of DNA double strand breaks (dsb) is analyzed by Pulsed Field Gel Electrophoresis (PFGE) using the CHEF system. Western blots will be done to determine the amount of topo I in the cells over the course of treatment. Our hope is that any potentiation seen will be greater in the two melanoma cell lines used than in the normal fibroblast cell line.

[GP01.168] Magnetic Resonance Image (MRI) Distortion and Specific Absorption Rate (SAR) in Biological Tissue

MRI has already been established as a routine technique in the diagnosis and control of clinical conditions, most remarkably cancer. Its main strength is its ability to accurately image soft tissue without contrast agents. In the seventies, the first MRI units used low radiofrequencies, of about 4MHz. In air, this corresponds to a wavelength of 75m. Even for tissue with permittivity of 68, the wavelength was still more than 1 meter. These conditions provide for a relatively uniform radiofrequency field inside the patient. With the purpose of improving signal-to-noise, the static field and thus the corresponding frequency have been escalating. Today, 64MHz units are commonplace, and plans are underway for the inception of 600MHz ones. Thus, the understanding of image distortion is critical to ensure reliable data to physicians. Likewise, SAR studies are important to comply with federal standards and avoiding tissue overheating. In this work we present a model of the human body and show profiles of SAR and image distortion for FID, Spin-Echo, and FLASH sequences.

Work supported by NIH grant CA77796-01.

[GP01.169] A Computational Model of Cerebral Blood Circulation: Application to the Study of Arteriovenous Malformations

A comprehensive computer model of the cerebral circulation, based on both hydrodynamics and electrical network analysis, was used to investigate the influences of arteriovenous malformations (AVMs) on regional cerebral hemodynamics. The basic model contained 114 normal compartments: 55arteries, 37 veins, 20 microvessel groups (MVGs), one compartment representing systemic and extracranial vascular resistance, and one representing the heart. Each MVG, which represented the arteriolar bed, consisted of 5,000 microvessels. Cerebral blood flow (CBF) autoregulation was simulated by a formula which determined the resistance and therefore the flow rate of the MVGs (arterioles) as a function of perfusion pressure. Models containing an AVM were constructed by adding an AVM compartment and its feeding arteries and draining veins. In addition to the basic model, AVM models were simulated with and without autoregulation and flow-induced conductance vessel dilation, respectively, to evaluate the contributions of these factors on cerebral hemodynamics.

[GP01.170] Electron Transport Along a Nanowire

Current technology has created nanowire structures with diameters as small as 10 nanometers and lengths as great as hundreds of microns. The performance of these nanowires may well differ substantially from the bulk properties of the materials from which they are fashioned. While the scale of these nanowires is quite small from a conventional electronics point of view, they still comprise thousands, if not millions, of nuclei and electrons. Hence, a full ab initio quantum mechanical calculation of these structures entails prohibitive computational expense. The prediction of the properties of these nanowires requires a new efficient approach for theorists to be able to provide insight that enhance the production cycle. We present in this work a new approach to this computational problem which is tested on a sample problem to assess its applicability to electron transport along nanowires. We compare quantum and semi-classical calculations on an atomic chain of twenty atoms length to understand the origin of the electron transport effects that are described by our approach.

[GP01.171] Assembly of Monodisperse Magnetic Nanocrystals into Nanocrystal Arrays

Ordered arrays of colloidal particles with varied dot size and lattice constants offer an ideal model for electrical, magnetic and optical study of nanomaterials. We will demonstrate that monodisperse cobalt- and nickel-based magnetic nanocrystals can be readily synthesized and subsequently induced to form 2-D and 3-D regular magnetic nanocrystal arrays. We further demonstrate that the distance between two particles can be adjusted by either thermal treatment or chemical ligand exchange. Low-temperature Coulomb Blockade of these magnetic dot arrays has been observed.

[GP01.172] Density of states of longitudinal electromagnetic modes in conducting or highly-dopped semiconducting superlattices

We model a system of alternating layers (made of metallic or highly doped semiconductors) as a system of wells and barriers. We used the hidrodynamic model and additional boundary conditions (ABC's) for the electron dynamics to find an equation of the longitudinal electromagnetic modes. We calculate the density of states of these modes.

[GP01.173] Interfacial Roughness and Effects of Thermal Annealing in CdS/CdTe Heterojunction Photovoltaic Materials

Grazing incidence x-ray scattering (GIXS) techniques have been employed to investigate the changes of interfacial roughness in CdS/CdTe heterojunctions under various heat treatment conditions. Both normal (CdTe/CdS/glass) and inverted (CdS/CdTe/glass) junctions were prepared by rf sputtering and subjected to thermal annealing at different temperatures. Thickness of the constituent layers, rms interfacial roughness parameters, and correlation lengths of height fluctuations were obtained as functions of the anneal temprature. These data are compared with previous measurements of angular dependence of x-ray fluorescence (ADXRF) using the same set of samples. Our results provide a direct evidence that the interfacial roughness increases and the lateral correlation length decreases with increasing anneal temperature as the interface becomes more blurred due to intermixing of Te atoms.

[GP01.174] Experimental Observation of Structural Transition with Increasing Cluster Size in Small Nano-Particles of Mo and W

We have produced small nano-particles of Mo and W using a CO2 laser multiphoton decomposition method in presence of photosensitizer SF6 and Ar gases. Products tinted black and were nano-size (2-10 nm). The particle size was controlled using a different density of metal vapor produced by decomposition of different amount of metal carbonyls in the gas cell. Ar gas also played a role in varying cluster size. We have recorded X-ray diffraction patterns of these nano-particles and observed for the first time a structural transition from amorphous structure to fcc structure with increasing cluster size. Particle size was obtained using TEM as well as using Scherrer's formula for FWHM of X-ray peaks. Cell constants of nano-size clusters were larger than those of bulk, which is understood to be due to surface tension. Our results support Tomanek et al's theoretical work who predicted a fcc structure for bcc transition metals when cluster size is small.

[GP01.175] Novel one-phase synthesis of thiol-functionalized Au, Pt, Pd,and Ir nanoparticles

Sub-micrometer size materials have attracted a remarkable academic and industrial effort of research due to their potential applications, among which fabrication of composite materials, information storage (opto-electronsics) and catalysis are of great importance. The synthetic route developed by Brust et al. is now commonly employed for the in situ preparation of inorgainc-organic core-shell composites. The method, however, suffers from certain limitations such as solubility issues, we intended to solve the problem by developing a new facile one-phase synthesis for thiol-functionalized Au, Pt, pd, and Ir nanoparticles using tetrahydrofuran as the solvent and lithium triethylborohydride as the reducing agent. For gold nanoparticles, high-resolution transmission electron micrograph of the drop-cast particle-film revealed the formation of truncated spherical particles of d=4 1 0.3 nm average size. The crystalline gold cores are surrounded with closely packed n-alkyl chains mainly in an all-trans conformation, adopting an orthorhombic packing as confirmed by FTIR spectroscopy. Particles are arranged in a discrete solid-like assembly with the inter-particle distance(center-to-center) of 5 nm and a constant edge-to-edge distance of 1 nm as shown by FFT-analysis. Characterizations like FTIR, TEM, XRD and UV have each been carried out for thiol-functionalized Pt, pd, and Ir nanoparticles.

[GP01.176] Quantum transport through atomic clusters

We have investigated quantum transport of atomic wires in which an atomic cluster is consisted of several Si or Al atoms. The atomic cluster is connected to two three-dimensional leads which are modeled by the jellium model. The quantum scattering problem is solved by combining the first principles ab initio method and a transfer matrix evaluation of the scattering matrix. The conductance of the atomic cluster is studied using Landauer formula for different cluster orientations. In the ab initio calculation, both LDA and GGA are used. A detailed comparison between the results using LDA and GGA is also given.

[GP01.177] The Atomic Structure and the Electronic Properties of Mutiwall Carbon Nanotubes

We report on the observation of the atomic structure and the electronic properties of multiwall carbon nanotubes (MWNT) using a scanning tunneling microscope (STM). Atomic resolution images show MWNT with different chiral angles as well as zigzag and armchair structures. A strong correlation is found between the atomic structure and the electronic properties in which the MWNT span the metallic- semiconductor regime.

[GP01.178] Radiative Exchange of Heat Between Nanostructures

Surfaces in close proximity in vacuum exchange heat through evanescent photon tunnelling modes as well as by freely propagating modes. These additional near field contributions to radiation scale with separation, d, between surfaces as d^-2 and are dominant at spacings d << \lambda _T, a typical wavelength at temperature T. We calculate a simple expression for the photon tunnelling in terms of surface reflectivity and find that there are drastic effects in many nanostructured systems, for example in the scanning tunnelling microscope.

[GP01.179] Self Assembly of Hetero-Structured Platinum Nanoparticles

Aqueous colloidal solutions of platinum nanoparticles with various shapes are easily synthesized by using hydrogen gas as the reducing agent. The shape of the resultant nanoparticles can be controlled by varying the initial ratio of the capping material to that of the platinum salt (Science, 1996, 272, 1924). High percentages of tetrahedral (with 111 faces), cubic (with 100 faces), and truncated octahedral (with both 111 and 100 faces) have been synthesized. However, the size distribution and the mixture of shapes found in these solutions has made the assembly of these nanoparticles on TEM grids into ordered arrays difficult. We have recently been able to overcome this problem by several modifications in the preparation of the TEM grids. The most notable of these enhancements is the addition of thiol to the colloidal solution which allows these nanoparticles to form self-assembled monolayers (SAMs) irrespective of the size or shape distributions. Hexagonally closest-packed (hcp) arrays are formed in hetero-structured solutions while cubic closest-packed (ccp) monolayers are found in solutions in which the cubic shape dominants. The interparticle interactions, distances, and solvent effects have been studied and will be discussed.

[GP01.180] Electron and Structure Dynamics in Gold Nanoparticles

The surface plasmon oscillation of noble metal nanoparticles presents a very sensitive and therefore excellent tool for probing the electron dynamics in these systems. The plasmon absorption of gold nanoparticles can be influenced by their size and shape as well as their structural composition in the case of mixed gold-silver particles (alloy nanoparticles). Furthermore, the chemical nature of absorbed molecules at the particle surface influences the absorption spectrum. This is modeled by Mie theory. However, all these parameters also play an important role in determining the fate of photoexcited electrons and their respective relaxation dynamics (electron-phonon scattering and electron-surface scattering in these confining systems). These electron as well as structure dynamics are investigated by femtosecond pump-probe spectroscopy. Furthermore, structural changes of the lattice induced by the intense laser heating are also studied on the same samples by transmission electron microscopy as a function of laser power.

[GP01.181] Time-Resolved FTIR and Visible Luminescence of Photoexcited Porous Silicon

Time-resolved FTIR and time-resolved photoluminescence of oxidized porous silicon (PS) are studied, in the 0 - 440 microsecond time domain, upon laser excitation at 355 nm. Laser excitation gives rise to an emission characteristic of the oxidized PS at 620 nm; two strong vibrational bleach modes at 1100 and 1235 cm-1 (in the region of the ground state Si-O-Si asymmetric stretching vibration frequencies); and a few new transient vibrational absorption bands in this region. Three characteristic lifetimes in the recovery of the bleach band at 1235 cm-1 are observed on the order of 1, 10 and 100 microsecond, similar to the observed decay times of the photoluminescence from PS. Our results suggest that the emitting centers in oxidized PS are trapped excitation sites in an inhomogeneously defected oxidized silicon surface.

[GP01.182] Multiphoton-Excited Luminescence from Diamond Nanoparticles and an Evolution to Emission Accompanying the Laser Vaporization Process

The photoluminescence (PL) from 100-nm diamond particles induced by infrared (IR) pulsed excitation [\lambda_exc. = 1064 nm (1.16 eV), \tau = 10 ns] was observed. The laser light intensity dependence of the PL yield shows that this is the five-photon absorption process (the total energy of excitation 5.8 eV). The PL results from structure defects, impurities and the graphite-like phase excited by a relaxation of the multiphoton (MP) - produced electron-hole pairs (the band gap of diamond Eg = 5.5 eV). By using the ultraviolet one-photon excitation [\lambda_exc. = 354.7 nm (3.48 eV), \tau = 10 ns], the same PL bands were observed in support of the correctness of interpretation. The typical Swan bands of electronically excited C_2 species dominate in spectra at the IR light intensity above the threshold for vaporization. It was concluded that the MP-excited PL precedes the emission accompanying the laser vaporization process.

[GP01.183] Coulomb Interaction of Two Electrons in the Quantum Dot Formed by the Surface Acoustic Wave in a Narrow Channel

Start your abstract by replacing this line with your text.

We develop a theory for the electron transport through a narrow channel within a two-dimensional electron gas (2DEG) produced by split gates on the surface of a piezoelectric material where a surface acoustic wave (SAW) is launched. A SAW is accompanied by a traveling electric field which drags electrons within the 2DEG and induces a current in the channel. There is a quantization of the induced current as the gate voltage varies. The lowest step can be explained in terms of a single-particle picture. The higher steps which are due to the transmission of at least two electrons can only be explained when the electron-electron interaction is included.

[GP01.184] Electronic Structure Calculation of Thin (GaAs)/(Ge_2) Superlattice Using EPM

Self-consistent (SC) method is the most reliable approach in calculating electronic band structure. The SC pseudopotentials calculation in the Density Functional Formalism in the Linear Density Approximation, however, is known to underestimate the bandgaps of many semiconductors and insulators. Such a calculation is also expensive and time-consuming for treating many atoms in a unit cell like the Superlattices (SLs). We show that by carefully choosing the bulk parameters, the much easier calculation using the Empirical Pseudopotential Method (EPM) also provides the similar trend of bandgaps for (GaAs)_n/(Ge_2)_n (with n = 1-3) with the SC calculation. For n = 1 the EPM gives no bandgap, similar to the previous SC calculation, while for n = 2,3 the bandgaps are direct and small.

[GP01.185] Pressler's Atom

I will present a novel theory concerning atomic structure, the position and nature of the electron inside the atom, and the nature of bonding, i.e., the covalent bond is described in terms of the interactions of atomic magnetic fields. Precise bond angles and distances of the molecule are considered. The coupling of the atomic magnetic field lines with the electron’s electric vector inside the diatomic hydrogen molecule is exhibited. This new concept is consistent with experimental evidence and adheres strictly to the valence-shell electron-pair repulsion (VSEPR) model presently used in chemistry for predicting the shapes of molecules and ions. In addition, I will explain the atomic model concept as being a true harmonic oscillator; periodic motion of the electron at resonant frequency produces radiation at discrete frequencies or line spectra because the electron is under the action of two restoring forces, electrostatic attraction and superconducting repulsion of the electron’s magnetic field by the nucleus. Valence is now consistent with electron shell structure and the model of the atom is complete.

[GP01.186] Ab Initio Calculations of HMX Reactivity

Using gas-phase and condensed-phase ab initio computational methods, we have explored the reactivity of the \alpha,\beta and \delta polymorphs of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The condensed-phase method employed is a novel, local-orbital basis set, parallel, density-functional code. By examining both the gas-phase and condensed-phase energetics we are able to identify important aspects of the various reaction mechanisms of HMX. In particular, we will present data for three primary mechanisms: the nitrogen-nitrogen bond scission; the carbon-nitrogen ring scission; and the HONO elimination.

[GP01.187] First-Principle Investigations of Electronic Structures and ^19F^\ast Nuclear Quadrupole Interaction in Fifth and Sixth Group Fluorides

The electronic structures of the fifth group fluorides NF_3, PF_3, AsF_3 and SbF_3 and sixth group fluorides SF_6 , SeF_6, and TeF_6 have been studied by the Hartree-Fock- Roothaan procedure with the aim to understand experimentally observed quadrupole interactions of the excited state ^19F^\ast [I=5/2] of the fluorine nucleus. The results provide good quantitative agreement with experiment and also explain the differing trends of variation in the quadrupole coupling constants, e^2qQ for the two series. Compassion will be made with the observed trends in the fourth- group which have been studied earlier using the Hartree-Fock-Roothaan procedure. The influence of many-body effects will be discussed.

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[GP01.188] Nonlinear electron response in atoms and molecules under intense laser

We report first-principles calculations of nonlinear electronic excitations in atoms and molecules driven by intense electromagnetic radiation. We implemented time-dependent density functional theory at the same level of sophistication and accuracy as state-of-the-art ground-state density functional theory (local density approximation for exchange-correlation, ab initio pseudopotentials, supercells, plane waves). In the case of intense infrared, we find that rare gases (He, Ar) yield a wide range of harmonics, all the way to x-rays, as observed experimentally. On the basis of such calculations we propose a novel way to generate harder x-rays from ions. In the case of intense ultraviolet, ultrashort pulses yield fascinating nonlinear response in low harmonics.

[GP01.189] Wave function optimization for many-body systems

We apply Green's function Monte Carlo methods to find the optimal parametrization of many-body wave function. The algorithm is derived from quantum simulation techniques and provides a way of discarding or replicating data sets with poor (high) energies or favorable (low) energies. The enhanced wave function is used in the calculation of electronic structure of molecules. We present the results of the wave function optimization method for LiH molecule. The application of the method to large systems is discussed.

[GP01.190] Ab initio Multiple-Scattering XAFS Debye-Waller Factors in Organic Systems

The molecular force field for the tetrachloroferrate (II) anion FeCl_4^2- is calculated using the ab initio density functional theory program DGauss. This force field is then used to calculate multiple-scattering x-ray absorption fine structure (XAFS) Debye-Waller (DW) factors via an efficient, finite temperature equation-of-motion (EM) method.(Phys. Rev. B\bf59), in press (1999). The EM approach improves on traditional isotropic models like the correlated Debye and Einstein models by including explicitly local near-neighbor interactions. The implementation of the method is compatible with the XAFS codes FEFF and FEFFIT, which also allows local microscopic force constants to be fitted to experimental spectra. These DW factors are used in FEFF8 XAFS calculations for tetramethylammonium tetrachloroferrate (II), [N(CH_3)_4]_2[FeCl_4]. A prescription for general DW calculations and transferability of force constants in organic and organo-metallic compounds is also discussed.

[GP01.191] Density Functional Investigation of the Electron Distribution and Hyperfine Parameters in [Fe(CN)_5NO]^3- and [Ru(CN)_5NO]^3-

The Discrete Variational method (DVM) in Density Functional Theory (DFT) was employed to investigate the electronic structure of the low-spin covalent complexes [Fe(CN)_5NO]^3- and [Ru(CN)_5NO]^3-. Spin-polarized calculations were performed to study the distribution of the unpaired electron: in both cases it is found to be localized mainly on the Nitrogen of the NO ligand, in an orbital formed by \pi-bonding between the 2p orbitals of N and the 3d_xz,yz orbitals of the metal. Electric field gradients were derived, with which were obtained Mossbauer quadrupole splttings; the result compares well with available experimental values in the case of [Fe(CN)_5NO]^3-. Elements of the hyperfine tensor A were determined; for the Fe complex, calculated values agree well with reported experimental data of \,^14N ESR on the nitrosyl ligand. For [Ru(CN)_5NO]^3-, predictions for the hyperfine parameters are made.

[GP01.192] Elucidation of the Properties and Dynamics of Molecular Imprint Polymers: Computational Experiments in Pursuit of Smart Nanomaterials

Molecularly imprinted polymers (MIPs) have a broad impact on many areas of science and technology which is evidenced by recent feasibility studies on the application of MIPs as: chemical and biomimetic sensors, separation media (resolution of racemates) and selective absorbents, antibody and receptor binding site mimics, enzyme mimics, and nano-reactors for selective organic synthesis. However the current practical applications of MIPs are limited because of slow binding kinetics, heterogeneous distribution of binding sites, modest binding capacities and selectivities, and the deleterious impact of water on binding. The objective of our work is to utilize both experiment and computation to rationally design the next generation of MIPs which may overcome these technical limitations for practical applications. Ab initio quantum chemical approaches (wavefunction and density functional theory) have been used to develop a detailed understanding of the type and strengths of chemical forces involved in binding of a MIP as well as to devise models for optimizing the binding kinetics. The effects of pressure, temperature, external radiation, and solvents on the binding kinetics are being investigated using the molecular dynamics (MD) and/or hybrid ab initio-MD methods. The computational experiments give insight into the functional groups that provide the correct balance of rigidity, flexibility, and accessibility of the polymer backbone as well as possibilities of using the hydrophobic interactions to enhance selectivity in aqueous environments. Results are presented on the nature and strength of the binding interactions between various functionalized monomers (derivatives of boronic and benzoic acid, aniline, and an amide) and toluene, nitrotoluene, nitrobenzene, dinitrotoluene (DNT), and trinitrotoluene (TNT). Optimization of the functionalized monomers based on this information has been confirmed through experimental studies using high pressure liquid chromotography (HPLC).

[GP01.193] Quantum Drops: A New Generation of Quantum Dots

Recent technological advances in the production of polymer particles suggests the feasibility of generating polymer nanoparticles from solution with discrete electronic structure in close analogy with quantum dots. We call these new particles quantum drops, which have controllable physical properties including radius, composition, chemical potential and magnetic properties. We consider the electronic and some magnetic properties of electrons confined on the surface of these spherical polymer nanoparticles, as well as their chemical potential, using a model Hamiltonian,semi-classical quantization and adiabatic switching.

[GP01.194] Hartree-Fock Investigation of Electronic Structures and Hyperfine Intraction in Nitrosylhemoglobin

The electronic structure and hyperfine interactions in nitrosylhemoglobin and the five-liganded nitrosylhemin have been studied in the past by the semi-empirical Self-Consistent Charge Extended Hückel (SCCEH) procedure with the principal aim of studying the structural changes associated with the R to T transition by the influence of environmental conditions. We are currently involved in first principle Hartree-Fock Roothaan investigations of the five-liganded nitrosylhemin andnitrosylhemoglobin, with similar aims, from a more quantitative point of view. Our results will be compared with the observed hyperfine constants in both systems and with the estimates from earlier semi-empirical SCCEH investigations.(S.K. Mun, Jane C. Chang and T.P. Das, Proc. Nat. Acad. Sci(USA), 76, 4842(1979))

[GP01.195] Hartree-Fock Investigation of Muon Trapping in the Chemical Ferromagnet 4-(p-Chlorobenzlideneamino)-TEMPO. (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yloxyl)

We have studied the muon trapping sites in the chemical ferromagnet 4-(p-Chlorobenzlideneamino)-TEMPO through energy minimization using the Hartree-Fock Roothaan procedure. Our investigations so far indicate that the muon is trapped at the Oxygen, Amino-Nitrogen and Chlorine sites. For the Chlorine site, the C-Cl bond distance after muon trapping is found to be rather large, suggesting that the influence of neighboring molecules will have to be considered. Results of our investigations on other likely trapping sites will be presented. The hyperfine fields at the various trapped muon sites will be evaluated and compared with experimental results from \muSR measurements.(Seiko Ohira, Master\primes Thesis, University of Tokyo (1997))

[GP01.196] Monte Carlo Simulations of Polarizable Liquid Water

The inclusion of electronic polarization within Monte Carlo simulations is hampered, relative to its inclusion within molecular dynamics simulations, by the need to fully determine the variables which specify the electronic configuration every time each molecule is moved, i.e. N times per cycle rather than once per cycle. Classical statistical mechanical Monte Carlo simulations of liquid water, with electronic degrees of freedom modeled by polarizable atomic sites and thus with components of the induced dipole vector requiring determination at every step, have been performed. Efficient procedures have been devised to determine this vector and a polarizable water model has been developed and tested on thermodynamic and structural properties of water.

[GP01.197] What does time-resolved terahertz spectroscopy say about solvation dynamics?

Time-resolved terahertz spectroscopy (TRTS) is an emerging technique in the study of solvation dynamics. This method uses sub-picosecond far-infrared pulses to probe a medium's low-frequency response to an ultrafast excitation pulse. Just as a medium's far-infrared spectrum yields its equilibrium polarization response function, a two-dimensional response function with an explicit dependence on time following the excitation pulse can be extracted from TRTS data. This response function, which can be simulated using phenomenological models or by molecular dynamics calculations, can provide insight into the evolution of solvent and solute mode properties during solvation.

[GP01.198] Stochastic Dynamics in Irreversible Non-Equilibrium Environments

A generalization of the generalized Langevin equation (stochastic dynamics) is introduced in order to describe dynamics that take place in changing environments. Such a scheme is needed to describe chemical reactions in environments that undergo isothermal compression. The friction kernel representing the solvent response is given a non-stationary form with respect to which the instantaneous random solvent force satisfies a natural generalization of the fluctuation-dissipation relation. Theoretical considerations, as well as numerical simulations, show that the dynamics of this construction satisfy the equipartition theorem beyond its equilibrium limits. The formalism is further extended to describe non-stationary environments in which the non-stationarity is induced by the macroscopic behavior of the ensemble itself, rather than an external force. Such a formalism lends itself to the dynamical study of the length distributions of growing polymers.

[GP01.199] Pump-probe Spectroscopic Study of Electron Transfer Pathways in Purple Bacterial Reaction Centers

Femtosecond transient spectra of purple bacterial reaction centers in the Q_X transition region were measured under various excitation conditions. Study has been focused on excitation wavelength dependence and excitation intensity dependence of electron transfer along the two potential pathways (referred to A and B branches). Low intensity excitation at various wavelengths, though generated different initial excited state, results in electron transfer only along the A branch. As the excitation intensity increased, up to 30observed. The formation of charge separated state on both sides show similar time constant of 1.3 ps. The B-side electron transfer most likely occurs through a double-photon process. The possible mechanism is discussed as a result of energy level balance change or secondary photochemistry.

[GP01.200] Ultrafast Multi-Color Pump-Probe Studies of Ferrous Cytochrome C

Ultrafast photophysical processes in horse heart ferrous cytochrome c have been studied by using the technique of femtosecond time-resolved spectroscopy. Both single-color and two-color pump-probe schemes are applied to measure the related kinetics. In the former case, cytochrome c has been excited and detected in Soret band, whereas in the latter case it has been excited in Soret band and detected in Q band. A number of subpicosecond and picosecond rates have been extracted from experimental data using maximum entropy technique. Based on this analysis, we suggest a model to describe the excitation and relaxation among the S_0, S_1 and S_2 states of cytochrome c.

[GP01.201] Morphing ab initio potentials for intermolecular forces: a systematic study of Ne--HF

A procedure for ``morphing'' an ab initio intermolecular potential energy surface to obtain agreement with experimental data is presented. The method involves scaling functions for both the energy and the intermolecular distance. In the present work, the scaling functions are parametrized and determined by least-squares fitting to the experimental data. The method is tested on the system Ne--HF, for which high-resolution infrared spectra are available. It is shown to work well even with relatively low-level ab initio calculations. Several basis sets are investigated at the CCSD(T) correlation level, including various aug-cc-pVnZ basis sets and the specially-tailored Ne--HF basis set of ONeil et al. All give good results after morphing, but the changes needed to match experiment are much smaller for the ONeil basis set. The use of MP2 calculations is also investigated: again, the MP2 potential is quite satisfactory after morphing, but requires much more modification than the CCSD(T) potential.

[GP01.202] Microwave Spectra and Nuclear Quadrupole Coupling in Trimethylgallium-Trimethylamine and Trimethylboron-Trimethylamine

The donor-acceptor complexes (CH_3)_3Ga-N(CH_3)_3 and (CH_3)_3B-N(CH_3)_3 have been studied by microwave spectroscopy in a supersonic jet. For the trimethylgallium adduct, an extremely dense spectrum resulting from both nitrogen and gallium nuclear hyperfine interactions together with satellite structure arising from methyl group internal rotation is observed. Similar results are obtained for the trimethylboron complex. The rotational constants for both species are consistent with the literature values of the Ga-N and B-N bond lengths (2.09 Å and 1.698 Å, respectively). The nuclear quadrupole coupling constants have been analyzed to provide new information about the electronic structure of the complexes. For (CH_3)_3^69Ga-^14N(CH_3)_3, a Townes and Dailey type analysis indicates that about 0.75 electrons are transferred from the trimethylamine to the trimethylgallium upon formation of the donor-acceptor bond, while for (CH_3)_3^11B-^14N(CH_3)_3 a value of 0.65 electrons is obtained. These results will be compared with similar measurements for related adducts of ammonia and trimethylamine.

[GP01.203] Long Range Collisional Stabilization and the Symmetry Constrained Dynamics of Group IIA Dihalide Complex Formation

A combination of single and multiple collision chemiluminescent studies and laser induced fluorescence spectroscopy has been used to (1) demonstrate the highly efficient collisional stabilization of electronically excited Group IIA dihalide collision complexes formed in direct M + X_2 \rightarrow MX_2 reactive encounters, (2) dilineate the first direct evidence for symmetry constraints associated with dihalide formation in the M + X_2 insertion process, (3) obtain the first discrete emission spectra for these dihalide complexes. These observations suggest that the extent of interaction of these high temperature molecular complexes, as they form, considerably exceeds that which we normally associate with collisional stabilization and energy transfer processes. High temperature molecules in electronically excited states or in high vibrational levels of their ground electronic states display a considerably extended interaction space. Neglect of these phenomena in models of chemical lasing, combustion, or propulasion systems renders these descriptions unrealistic.

[GP01.204] Vibrationally Resolved Negative Ion Photoelectron Spectroscopic Studies of Group V Transition Metal Trimer and Tetramer Monoxides

Negative ion photoelectron spectra are reported for the M_3O and M_4O clusters of V, Nb, and Ta. The anions were prepared in a liquid nitrogen cooled flowing afterglow ion-molecule reactor equipped with a metal cathode cluster source. The spectra of the mass-selected clusters were obtained at 488 nm with 5 meV resolution. All six clusters have electron affinities within the 1.2-1.6 eV range. The transitions to the neutral molecule ground states display short vibrational progressions indicative of the detachment of a non-bonding electron from the anion. Each spectrum displays a high frequency (670-750 cm^-1) mode, which is assigned as a metal-oxygen stretch, and one or more lower frequency (150-400 cm^-1) modes. Additionally, the spectra reveal vibrationally resolved transitions to low-lying excited electronic states. Comparison of these data to computational results in the literature provides insight into the likely bonding modes of the oxygen atom to these trimer and tetramer early transition metal clusters.

[GP01.205] Vanadium Clusters: Reactivity with CO, NO, O_2, D_2, and N_2

We have investigated the size-dependent reactivity of vanadium clusters, V_10--V_60, with CO, NO, O_2, D_2 and N_2 by using a laser-vaporization source, low-pressure reaction cell and photoionization time-of-flight mass spectrometer. The reaction probability in a collision between a cluster and a gas molecule was determined for each gas for the first and second molecule. The reactivity was measured at two different cluster-source temperatures: room temperature (RT) and liquid-nitrogen temperature (LNT). We find that vanadium clusters react readily with all molecules investigated. With CO, NO and O_2, the reaction probability is high and exhibits relatively weak size dependence. With D_2 and N_2 the overall reaction probability is lower, and significant size variations are observed over the whole size range investigated, with coinciding minima and maxima for the two molecules. Upon cooling of the cluster source to LNT, the reactivity increases, but the overall size dependence is very similar to that observed at RT. By analyzing the total relative abundance of each cluster size as a function of reaction-cell pressure, we find that reaction of V_n with CO, NO and O_2 results in size-dependent cluster-product fragmentation of clusters in the size range n\approx10--20, most likely, by metal-atom loss.

[GP01.206] N_2 on Tungsten Clusters: Molecular and Dissociative Adsorption

We have studied the size-dependent reactivity of W_10--W_60 with N_2 under single-collision-like conditions by using a laser-vaporization source, a low-pressure reaction cell and a laser-ionization time-of-flight mass spectrometer. The reaction probability with the first and second N_2 molecule was measured at two different cluster-source temperatures: room temperature (RT) and liquid-nitrogen temperature (LNT). To get an indication of the bond strength of N_2 on W_n, we heated the cluster products after reaction through irradiation with 4.02-eV photons from a XeCl excimer laser and checked for consequent desorption of adsorbate atoms or molecules. For the LNT clusters, heating with laser light caused a substantial decrease in the abundance of reaction products with nitrogen, whereas no significant change in the abundance of W_nN_2 was observed for the RT clusters. This indicates that a proportion of the N_2 is relatively weakly bound onto the LNT clusters, whereas on W_n produced at RT, only the strongly bound state/states of N_2 exist. Based on comparisons with the N_2-adsorption on W bulk surfaces, we conclude that the weakly and strongly bound state represents molecularly and dissociatively bound N_2, respectively.

[GP01.207] Vibrationally Resolved Photoelectron Spectra of Niobium Cluster Anions

Vibrationally resolved negative ion photoelectron spectra are reported for Nb_n (n=2,3,4,6 and 8) and Nb_3H(D). The corresponding anions were prepared in a liquid nitrogen cooled flowing afterglow ion-molecule reactor equipped with a niobium cathode discharge source, and 488 nm spectra of the mass-selected clusters were obtained at 5 meV resolution. The Nb_8 photoelectron spectrum is surprisingly simple, and displays a single vibrational progression in a mode whose frequency is 185 (±15) cm^-1 in the neutral molecule. The photoelectron spectra of Nb_3 and Nb_3H display similar electron affinities, frequencies and normal mode displacements for their dominant vibrational progressions, suggesting that the Nb_3 cluster is only mildly perturbed by H addition. Partially resolved vibrational features in the photoelectron spectra of Nb_4 and Nb_6, and the relatively complex spectrum of Nb_2 which shows transitions to the ground and several excited electronic states of the neutral dimer, are also reported.

[GP01.208] Polarity and Microsolvation of Partially Formed Bonds

Lewis acid-base complexes with a partially formed dative bond exhibit pronounced changes in structure upon crystallization [1]. This sensitivity to a local environment can be understood in terms of the response of the molecular dipole moment to the net electric field produced by neighboring molecules in the solid. In this work, we examine the effect of a single nearest neighbor on the structure of a partially bonded molecule and investigate the dipole moments of partially bound species.

Microwave spectra of eight isotopomers of HCN-HCN-SO_3 have been observed. The data indicate a 0.12 Å contraction of the N-S bond relative to that in HCN-SO_3. Stark effect measurements on a series of SO_3 complexes are also reported and indicate a substantial increase in induced moment as dative bond formation proceeds. This result is discussed in connection with both microsolvation and the origin of the large gas-to-solid structure changes characteristic of partially bound systems.

[GP01.209] Photoemission from gold nanorods

We observe strong apparently size-dependent photoemission from gold nanorods solutions of different average aspect ratio. The gold nanorods are prepered via an electochemical method as descibed by Yu et al (J. Phys. Chem. B. 1997, 101, 34, 6661). We found that the life times of this emission depend on the average aspect ratio of gold nanorod solution. Moreover, the temperature dependence of the photoemission efficiency and life times have been studied for gold nanorod solution of 100 nm average length and 20 nm width.

[GP01.210] Photoelectron Spectroscopic Studies of the Reactions of V and Nb Atomic and Cluster Anions with Benzene and Ethylene

A flowing afterglow ion-molecule reactor has been used to study the interaction of small vanadium and niobium atomic and cluster anions with benzene and ethylene. Mass selected anionic products have been further investigated by photoelectron spectroscopy. The spectra of V(C_6H/D_6)^- and Nb(C_6H/D_6)^- produced by reaction with benzene are quite similar, each displaying clearly resolved vibrational progressions in the totally symmetric C-H out-of-plane bending and metal-ring stretching modes of an \eta^6 half-sandwich complex. Reaction of niobium atomic and cluster anions with ethylene yields NbC_2H_2^-, NbC_4H_4^- and NbC_6H_6^- as the major anionic products, despite the low initial abundance of the atomic anion. The NbC_6H_6^- anions yield an identical photoelectron spectrum to that obtained on reaction with benzene, demonstrating that these dehydrogenation products also have the \eta^6-benzene structure. The spectrum of V_2(C_6H_6)^- and other anions produced on reaction with benzene are also reported.

[GP01.211] Fragmentation of molecular clusters following electron impact.

A variety of techniques have been applied to study the interaction of electrons with beams of CO_2 and N_2O molecules which are heavily clustered following supersonic expansion from a room temperature, pulsed valve source. Fragmentation of the clusters is monitored by detecting excited fragments directly or via the vacuum ultra-violet photons which they emit as they decay. Detailed comparisons are made between data sets obtained for clustered and non-clustered beams. The evidence obtained points to the conclusion that, for these parent gases, the excited molecules are essentially decoupled from the clusters during fragmentation. Significant differences are highlighted between data obtained with molecular based clusters and those obtained using rare gases.

[GP01.212] Perpendicular bands of CH_2 X molecules as contributors to the diffuse interstellar bands

We have been studying the relatively regular Herbig diffuse interstellar band (DIB) group near 6800 Åas it may be due to perpendicular, rovibronic bands of a molecule or ion of the form CH_2 X (Glinski and Nuth, 1995 PASP 107, 453). The spacing of about 18 cm^-1 and the intensity alternation is entirely consistent with the rotational constants and ortho/para ratio corresponding to the H-C-H moiety. We have been modeling the spectrum with various carbon chains as the X. We will show results of our preliminary models of the perpendicular band group for H_2 C_5. We use recently measured B rotational constants of McCarthy et al. (1997 ApJSS 113, 105) and estimable A constants to synthesize the spectrum. We note that there is relatively little leeway for varying the A and B constants in these types of molecules because they are rather rigid. We are continuing to refine our models of these groups of bands along with their single, parallel band counterparts. These multiple bands will serve as a strong test of any spectroscopic fit. Our results support the idea that a family of carbon chain molecules may account for many of the weak DIBs in accord with the work of John Maier`s group (1998 ApJ 506, L69).

[GP01.213] Surface modification of PE film: Synthesis of PE-S-Ph-NH2 and its derivatives

Amino terminated PE (PE-S-Ph-NH2) film was carried out by two step reactions, bromination reaction following by substitution reaction. Brominated PE (PE-Br) film was obtained from gas phase bromination of the PE film. Advancing water contact angle dropped from 1000 to \sim900 after bromination. X-ray photoelectron spectroscopy (XPS) analysis showed 14% bromine atom incorporated. Substitution reaction of PE-Br film with p-aminothiophenol was carried in ethanolic solution yielding PE-S-Ph-NH2. XPS analysis showed 6% nitrogen atom incorporated. chemical reactions with p-nitrobenzaldehyde following by hydrolysis yielded amine group density of \sim2 molecules/nm2. Derivatization of PE-S-Ph-NH2 can be done using amino group chemistry, such as trifluoracetylation, diazotization, etc.

Part G of program listing