Session DP01 - Poster Session I.
POSTER session, Monday morning, March 22
Exhibit Hall, GWCC

[DP01.01] Photoionization of H^- from 0.78 to 12.8 eV

The photoionization cross section of H^- is calculated with a saddle-point complex-rotation method. Two Feshbach resonances and one shape resonance near the n=2 threshold are measured to high precision in a recent experiment(H. H. Andersen, et al., Phys. Rev. Lett. 79), 4770 (1997). These Feshbach resonances are extremely narrow. To make an unambigorous comparison with experiment, we have convoluted the theoretical cross section with a Gaussian profile using experimental resolution (0.18 meV) for FWHM. Our results for the shape resonance are also compared with that of Lindroth(E. Lindroth, private communication.). These cross sections are studied using length, velocity, and acceleration gauges. For most energies, the results from the three gauges agree extremely well. The Feshbach resonances below the n=3 threshold are also investigated.

[DP01.02] Photoionization of lithium between the Li^+ 1s2p\,^1P and 1s3s\,^3S thresholds

The photoionization of lithium from 1s^22s is studied for photon energy between 70 and 74 eV. The photoionization cross section is calculated with a saddle-point complex-rotation method. The agreement between the results from length gauge and velocity gauge are well within 1 %. These results are compared with the theoretical and experimental data in the literature. Many resonances are obtained in this energy region. The predicted Li [1s(3s3p)^3P]\,^2P^o resonance energy and width are 71.163 and 0.07475 eV, respectively. The predicted Li [1s(3s3p)^1P]\,^2P^o resonance energy and width are 71.512 and 0.1656 eV, respectively. The partial Auger widths of the ^2P^o resonances are also studied.

[DP01.03] Photoionization of beryllium from the 1s^22s2p\;^1P^o

The photoionization cross section (PICS) of Be from the 1s^22s2p\;^1P^o is studied with the saddle-point complex-rotation method for photon energies from 23 to 117 eV. A full-core plus correlation wave function is used for the initial states. The nonrelativistic energy of this wave function is -14.472\,984 a.u. For the resonances in the continuum, the energy and width for the singly core-excited Be 1s2s2p^2 ^1S, ^1P, and ^1D states are calculated to high precision. The Auger decay branching ratios of these states are studied to check the spin alignment and charge density overlap theory recently proposed by Chung (K. T. Chung, to be published). We found that the PICS to the ^1D continuum is about three times that to the ^1S continuum near the Be^2+ 1s1s threshold. The 1s2s2p^2 ^1D width is about 1.3 times that of 1s2s2p^2 ^1S. Its peak PICS is about four times that of 1s2s2p^2 ^1S.

[DP01.04] Photoionization of beryllium and Auger branching ratios of 1s2s^2np ^1P^o

The photoionization cross section (PICS) of Be from the ground state is studied with the saddle-point complex-rotation method for photon energies from 25 to 122 eV. A full-core plus correlation wave function is used for the ground state. The nonrelativistic energy of this wave function is -14.667\,033 a.u. For the resonances in the continuum, the energy and width for the singly core-excited Be 1s2s^2np ^1P^o (n=2, 3, amp; 4) are calculated to high precision. The PICS are studied with single open channel approximation as well as fully coupled open channels. The Auger branching ratios of these states are calculated. These results are compared with the previous theoretical and experimental data in the literature. Our Energy and width of 1s2s^22p are 115.5 eV and 38 meV which agree with those of Caldwell et al ( C. D. Caldwell, et al), Phys. Rev. A 41 542 (1990)..

[DP01.05] Absolute photoionization cross sections of positive atomic ions: C^+.

Absolute photoionization cross sections of positive atomic ions of astrophysical importance have been studied, including C^+ in the 2s^22p ^2P ground state. Such cross sections are essential for astrophysical modelling, which so far has relied on theoretical predictions from the OPACITY(The Opacity Project, Institute of Physics, Bristol 1995, Vol. 1.) project or the OPAL project (see ref. 2). The experiments were performed at the new undulator beam line at the Aarhus storage ring ASTRID, by merging a 2keV ion beam with a photon beam over a distance of 60cm and recording the production of double charged ions. The C^+ spectra were recorded in the energy range 24 - 31eV with a photon flux of approximately 10^12 photons/s and a 70nA ion beam. Comparison between the experimental and theoretical data exhibits significant differences which can be attributed to the neglect relativistic effects in the calculation(Yan and Seaton J. Phys. B. 20 (1987) 6409.).

[DP01.06] Auger Decay of the Argon Photoexcited 2p^-1n\ell Rydberg Series

The 2p^-1ns(nd) inner shell photoexcited resonance states in argon are studied using a combination of optical potential, multichannel quantum defect theoretical, and R-matrix methods. Optical potential inclusion of the infinite number of core decay channels correctly accounts, in an implicit manner, for the physical broadening of resonances within the 3p^-1 and 3s^-1 cross sections, yet highly asymmetric resonance features remain. Comparison with experimental results is made, showing good qualitative agreement in the resonance profiles. The quantitative differences are due to unconverged theoretical resonance energies and to discrepancies in the background cross sections. Fine structure resolved cross sections and angular distribution parameters are discerned in the present calculations, revealing major qualitative departure from nonrelativistic predictions; these features should be observable in high resolution measurements.

[DP01.07] The dynamics of L-shell ionization in magnesium

Photons and electrons above \sim50 eV can ionize Mg and Mg^+ respectively by the ejection of a 2p electron; at higher energies \sim90 eV a 2s electron can be ionized. New R-matrix calculations are reported on these L-shell processes in the photoionization cross sections of ground state and metastable states of Mg, and in the electron impact ionization cross section of Mg^+. Resonances arising from interference of inner-shell and outer-shell channels are quantified.

[DP01.08] Correlational Oscillation in the Non-Dipole Anisotropy Parameters of Photoelectrons

The parameters Gamma and Eta, which determine the nondipole correction to the near threshold photoelectron angular distributions in noble gas atoms are calculated in Hartree- Fock (HF) one-electron approximation and with account of multielectron correlations in Random Phase Approximation with Exchange (RPAE). Additional to the 1s in He, the outer np and the subvalent ns subshells in Ne, Ar and Xe are considered. We find that HF results deviate from those of the much simpler one-electron approximations and from the RPAE calculations that HF is insufficient for Ne, Ar and Xe. Correlations are particularly important for the dipole transitions, where both intra- and inter-subshell interactions must be taken into account. In quadrupole transitions inter-electron correlations are also important. In Ne and Ar accounting for the interaction within the groups of outer np and ns electrons and between the groups is sufficient, while in Xe the influence of the 4d^10 subshell is huge, resulting from its dipole giant resonance and quadrupole excitations. In summary, the inclusion of RPAE effects results in at least one additional, in comparison with HF results, oscillation in the Gamma and Eta parameters as functions of the photoelectron energy. This predicted peculiar behavior calls for experimental verification.

[DP01.09] A unified view of high energy photoionization of two--electron atoms through photoabsorption in terms of singularities of the Coulombic potential

We describe within a unified approach the single and double photoionization by photoabsorption of two electron atoms at high photon energies ømega (but still ømega \ll m). The treatment is nonrelativistic and the three bodies interact by Coulombic interaction. The singularities of the interaction (e--e and e--N) determine the high energy behavior of the processes and in the case of double ionization explain the shake--off and quasi--free contributions at high energies. Since photoabsorption at high photon energies requires at least one large outgoing electron momentum, the analysis is equivalent to the analysis of the asymptotics of Fourier transforms. The asymptotic behavior is determined from the singularities of initial and final state three--body wave functions and interactions. These in turn follow from the singularities of the interaction between particles. Results are gauge and form independent, but dependence on the quality of initial and final wave functions varies with gauge and form. We present exact results for properties of total cross sections in the high energy limit for both single and double ionization from ground and excited states.

[DP01.10] Breakdown of independent particle approximation in high energy photoionization

In independent particle approximation (IPA) the behavior of the cross-section for photoionization of a state of angular momentum l in the high energy nonrelativistic limit is known to be of the form \sigma _l=Dc_l/ømega^(l+7/2), where ømega is the photon energy, and D is an l independent factor which approaches its high energy limit slowly. The value of c_l will depend on the choice of IPA potential. We show that inclusion of IPA breaking effects changes the functional dependence for l > 0 into \sigma _l=Da_l/ømega^9/2, thus modifying the value of c_l for l=1 and changing the form for l\geq 2. We estimate that for large nuclear charge Z inclusion of IPA breaking effects in ionization of 2p states is manifested as a multiplicative factor (1+1/Z) in the coefficient c_1. For all l\geq 2 a_l\sim Z^5 in this limit, while it is known that c_l\sim Z^5+2l. Using a perturbative treatment for the IPA breaking effects of the electron-electron interaction, we have found explicit expressions in terms of the matrix elements of IPA functions. We apply our results to the case recently studied experimentally of ionization of the 2p state of Ne by photons of energy ømega =1 keV (E. W. B. Dias et al, Phys. Rev. Lett. 78, 553 (1997).).

[DP01.11] RPA studies of nondipolar angular-distribution asymmetry parameters in the n=2 shell of neon

Correlation effects in the dipole--quadrupole angular-distribution asymmetry parameters \delta and \gamma for closed-shell atoms are evaluated in the random-phase approximation. Relativistic (RRPA) and nonrelativistic (RPAE) calculations of the dipole asymmetry parameter \beta and the nondipolar parameters \delta and \gamma are in close agreement with one another for the outer shells of neutral atoms. The RPA angular-distribution parameters are compared with independent-particle model calculations(J.W. Cooper, Phys.\ Rev.\ A47), 1841 (1993). and with experimental angular-distribution asymmetry parameters(O. Hemmers et al.), J. Phys.\ B30 1727 (1997). for the n=2 shell of neon measured in the 250-1200 eV photon energy interval.

[DP01.12] Investigations of the Triple Photoionization Threshold Law

We have measured the triple-photoionization cross-section of lithium near its threshold (203.5~eV) employing an ion time-of-flight spectrometer and monochromatized synchrotron radiation between 205~eV and 240~eV. Since lithium is a three-electron system, Auger decay or autoionization processes cannot contribute to the triple-ionization cross-section. Therefore lithium is well-suited to study the threshold behavior of the triple photoionization process. Such a study was undertaken for the case of neon and atomic oxygen(J.A.R.\ Samson and G.C.\ Angel, Phys.\ Rev.\ Lett.\ 61), 1584 (1988).. However, this study was limited to up to 9~eV above the corresponding thresholds. While our study confirms the observation of Ref.~[3] for the first 5~eV, we determined a different power law above 5~eV which is valid up to about 25~eV above threshold.

[DP01.13] Ionization and pair creation with 1.33 MeV photons

We present inner-shell photoionization and pair production measurements for 1.33 MeV photons impinging on atomic targets. A high intensity photon beam is produced by collimation of a 5000 Curie ^60Co source. Using coincidence techniques we measure the fraction of events in which an electron-positron pair is produced simultaneously with the ionization of the target. The probability for this simultaneous event to occur is compared to the product of the probabilities of the single events in order to unravel correlation between ionization and pair production. The preliminary experimental results will be presented. Work supported by the U. S. Department of energy under contract No DE-AC-03-76SF00098.

[DP01.14] Resonance Auger spectra of C~1s^-12\pi_u excited CO_2

Vibrationally resolved electron spectra of CO_2 have been measured following excitation at photon energies across the broad C1s^-12\pi_u resonance (h\nu \approx 291~eV). Electron spectra were obtained using photons from beamline 10.0.1 at the Advanced Light Source with a gas cell and Scienta SES-200 hemispherical electron energy analyzer mounted on a rotatable chamber to permit measurement of electron angular distributions. Spectra measured at the peak of the resonance are consistent with previous low resolution results(T.X.~Carroll and T.D.~Thomas, J.~Chem.~Phys.~94), 11 (1991). where it was observed that the spectator Auger decay process is favored. In the region of the 1-hole states, it was also found that only the 1\pi_u^-1 final state exhibits any significant increase in intensity. The vibrational levels populated in the 1\pi_u^-1 band are found to depend upon the photon energy used, with lower levels favored at low photon energies and higher levels at higher photon energies across the band. Spectra measured across the breadth of the C1s^-12\pi_u resonance will be presented along with asymmetry parameters at selected photon energies.

[DP01.15] Scattering of atomic beams from finite, weakly interacting Bosonic systems.

We present results of a calculation of the scattering cross-section of a confined, weakly interacting system of Bosons which is exact within the Bogoliubov approximation. The results display evidence of the existence of a condensate mediated atomic transmission process similar to that originally postulated to exist in strongly interacting superfluid helium (J.W. Halley et al, PRL 71,2429 (1993); A.K. Setty et al, PRL 79, 3930 (1997)). In the calculation reported here the bosons are confined by a single particle potential close to that for a square well with the unphysical edges smoothed out. The scattering and bound states are determined for a specific well depth and a range of interaction by solving the Gross-Pitaeveskii and Bogoliubov equations numerically. The results show transparency effects in the scattering cross-section strongly suggestive of the condensate mediated process. Furthermore, preliminary studies of scaling of the transmission time with system size have shown characteristics associated with the same effect. These results suggest possible experiments in weakly interacting bose condensed alkali systems.

[DP01.16] Multi-electron excitations during photoionization of light atoms

One of the more obvious manifestations of the dielectronic interaction in light atoms is the simultaneous excitation of several electrons after the absorption of a single photon. Since the absorption of a photon is a single-electron operator, a multi-electron response must involve some transport of energy from one electron to the other. In the present contribution, we will show several manifestations of a multi-electron response: the excitation of high-lying doubly-excited states of He, double photoionization of H^-, He and Li, and triple photoionization of Li. Where possible our theoretical results will be compared with experimental and other theoretical results.

This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences and used resources of the National Energy Research Scientific Computing Center.

[DP01.17] Time Dependence of Photoelectron Angular Distributions

We predict that under certain circumstances shape resonant photoelectron angular distributions can vary with time. The time-dependence traces to the different time delays experienced by the alternative orbital momentum components of the photoionization amplitude and the coupling of these components during the escape of the photoelectron. To explore this effect we have developed a two orbital momentum model in terms of the interaction time with the light, the ratio of the transition dipole moments, the time delays of each channel, and the energy dependence of their coupling.

[DP01.18] Differential Double Ionization of H^- by Compton Scattering

Double ionization of negative hydrogen, H^-, is studied for Compton scattering differential in energy transfer. The cross sections for double and single ionization of H^- are calculated for Compton photons up to 100 keV. The ratio of double to single ionization is obtained as a function of energy transfer. Comparison is made with previous studies with helium. It is found that the non-equivalency of the two electrons in H^- influences substantially the cross sections, and hence the ratio. The large orbit of the outer electron increases dramatically the range of angular momentum transfers compared to helium. It also appears that the threshold behavior of the cross sections differs from helium. Other differences and similarities are observed and will be discussed including the integrated ratio, the transition from two-body to three-body dynamics in the binary encounter region, and the role of the relative strengths of electron-electron and electron-nucleus interactions.

[DP01.19] Relativistic Effects in the Photoionization of Ne-like Iron

We demonstrate that it is not always necessary to resort to the more complicated full Breit-Pauli R-matrix method in order to include relativistic effects in photoionization calculations. To allow for the fine structure splitting of channels in the photoionization of Fe^16+, we have performed two sets of calculations. The first combined a LS R-matrix calculation with a LS-JK frame transformation, using multichannel quantum defect theory (MQDT). The second used a relativistic random phase approximation (RRPA) based on the Dirac equation. Both methods give resonant photoionization results nearly identical to those from a full Breit-Pauli calculation. An accurate treatment of fine structure splitting in Fe^16+ is necessary since in the inverse process of photorecombination, the low temperature rate coefficient is dominated by the 2p^5(^2P_1/2)nl dielectronic recombination resonances.( D. W. Savin, et al.), Astr. Jour. 489, L115 (1997).

[DP01.20] Measurement of Positions, Relative Intensities, Air-Broadening, Pressure Shifting and Line Mixing Coefficients in the \nu_3 Band of ^12CH_3D

A multispectrum nonlinear least-squares fitting technique has been used to determine accurate line positions, relative intensities, air-broadening coefficients and air pressure-induced shift coefficients for more than 360 transitions in the \nu_3 band of ^12CH_3D in the spectral region between 1154 and 1430 cm^-1. A total of 11 absorption spectra were used in the analysis. Low-pressure spectra with 1 to 3 torr of 12CH3D as well as lean mixtures (\sim 1%) of ^12CH_3D in dry air (100 to 400 torr) were used in recording the data. All spectra used in this study were obtained at room temperature using the McMath-Pierce Fourier transform spectrometer of the National Solar Observatory on Kitt Peak. The analysis included measurements for transitions up to J\prime\prime=17 and K\prime\prime=17. The broadening coefficients range from 0.0155 to 0.0726 cm^-1 atm^-1 at 296 K and the pressure shift coefficients vary from -0.0086 to +0.0058 cm^-1 atm^-1 at 296 K. Weak line mixing effects were observed in a few high J lines in the K\prime\prime=3 transitions and the line mixing coefficients were determined for the A^+A^- (A1A2) split components. At low to medium values of J the A^+A^- splitting were extremely small and the two components were practically unresolved. Variations of the measured parameters with the A- and E- symmetry species and the rotational quantum numbers of the transitions involved are discussed. The results are compared with the few experimental results reported in the literature and the theoretically calculated values in the HITRAN 1996 line parameters database.

[DP01.21] ESR Study of Fatigue of Polymers

In the course of polymer fatigue significant changes in the order of chains appear. These changes influence the molecular dynamics of polymes. Because of free volume redistribution some degrees of freedom of motions arise or disappear.By using gamma radiation we can generate a high concentration of free radicals in polymers. These free radicals can decay only via a convenient transport mechanism.The decay of free radicals can proceed only if two radical centres approach each other within an appropriate distance.A great variety of molecular motions depending on temperature, pressure and chain order,are manifested in polymers. A small change in molecular dynamics may have a considerable influence on the kinetics of free radical decay.The free radicals generated by gamma radiation can thus serve as molecular labels for monitoring the changes in molecular motion due to the fatigue of a polymer.During the fatigue of material the order of polymer chains increases, the free volume decreases and thus the stability of free radicals increases,the decay of free radicals is inhibited.

[DP01.22] Precision Measurement of Dielectric Relaxation in Water at Microwave Frequencies

A cylindrical microwave resonant cavity in TE011 mode is used as a probe to study the dielectric behavior in a sample of dionized distilled water. The dielectric response of water is studied as a function of temperature at microwave frequencies of 10.2, 10.6, and 11.4 GHz. A very sensitive thermal bath is used to control the temperature of the molecule under study near its phase transition. The thermocouple making a contact with the material is controlled through a computer using the technique of computer interfacing. A significant number of data points are taken during the dielectric relaxation of the material as it goes through a phase change. Slater's perturbation equations are used to calculate the real and imaginary parts of the complex permittivity. The relaxation times are calculated using Debye's theory of polar molecules.

Supported by the Grants and Research Funding Committee at Southeast Missouri State University.

[DP01.23] Improved microwave spectroscopy of high-L Rydberg states of H_2

Microwave/optical spectroscopy with a fast beam of H_2 has been used to determine all the fine structure intervals between n=9 and 10 Rydberg levels of H_2 with L of 5 or more which are bound to the (\nu=0, R=1) state of H_2^+. This is a much more extensive pattern than was obtained in an earlier study of this type,(W.G. Sturrus, et. al., Phys. Rev. A 44, 3032 (1991)) and should lead to much improved determinations of the quadrupole moment and polarizabilities of the ion core.

[DP01.24] Microwave spectroscopy of G-H and H-I fine structure intervals in helium, n=7, 9, and 10

New measurements of the G-H and H-I fine structure intervals in n=7 and 9 of helium show discrepancies with theory(G.W.F. Drake, in Atomic, Molecular, and Optical Physics Handbook, ed. G.W.F. Drake, AIP Press (1996), p. 154) which are similar in size but of opposite sign to discrepancies previously noted in n=10.(E.A. Hessels, et. al. Phys. Rev. A 46, 2622 (1992)) A remeasurement of the 10 G-H interval confirms this difference. This pattern of discrepancies could help to clarify the limitations of current theory for this fundamental system.

[DP01.25] Microwave spectroscopy of helium-like Rydberg states of HD

Six fine structure intervals in high-L, n=9 and 10 Rydberg states of HD bound to the \nu=0, R=0 ground state of HD^+ have been measured with the same microwave/optical technique used previously to study analogous states of H_2 and D_2.(P.L. Jacobson, et. al., Phys. Rev. A56, R4361(1997), A57, 4065(1998))The measured HD intervals reveal substantial perturbations due to the permanent dipole moment of the HD^+ core. These are especially noticeable in states with L of 6 or less, which are coupled to \nu=0, R=1, n=6 perturbing levels. These initial interval measurements are largely consistent with the long-range picture of Rydberg structure. With continued study, we hope to understand the structure in much finer detail.

[DP01.26] Convergent Upper and Lower Bounds on the Eigenenergies for the Hydrogen Atom in a Constant Magnetic Field

The hydrogen atom in a constant magnetic field has been studied for many years both because of its astrophysical applications and as a simple, yet challenging example of a non-separable system. The Fock expansion\footnote Yu P. Kravchenko, M.A. Liberman and B. Johansson, Phys. Rev. A 54, 287 (1996) is a solution to the Schrödinger equation for this physical system. The boundary conditions for the Schrödinger equation can be satisfied by using the Raleigh-Ritz variational principle which yields upper bounds for the eigenenergies. The Schwinger variational principal can also be used to satisfy the boundary conditions and yields lower bounds for the eigenenergies. Therefore, by combining these two methods, the exact eigenenergies can be bracketed from both above and below. Furthermore, because the number of terms which survive in both calculations is finite and depends only upon the number of terms in the Fock expansion, these methods can in principle produce results of arbitrary precision.

[DP01.27] Novel absorption cell design for precision measurements of molecular absorption cross-sections in hot sodium + noble gases mixtures.

We present a novel design of an absorption cell to contain sodium and other alkali vapors mixed with noble gases at different pressures and temperatures. Most absorption spectroscopy experiments have been performed in heat pipe cells, where the absorption length is often poorly defined, or in completely sealed cells with hot windows that did not easily allow changing the gas mixtures. In our absorption cell we use hot sapphire windows to define the homogeneous absorption path and high temperature valve, kept at the same temperature as the cell itself, to introduce different noble gases. A separate small sodium reservoir, maintained at a lower temperature, is used to control the sodium vapor pressure independently of the cell temperature. Using this cell and a 3m Czerny-Turner spectrometer we will measure absolute values of the molecular absorption cross-sections in the wavelength range 400 - 800 nm at high resolution. To determine accurately the sodium concentration in the mixture, we place the cell in the test arm of a Mach-Zender interferometer and use the ‘hook’ method in the vicinity of the Na line. The concentration of noble gas is determined from pressure and temperature measurements. Supported in part by the NSF and OSRAM Silvania.

[DP01.28] Frequency stabilization of a mode-locked bi-directional Ti:sapphire laser for high-resolution differential intra-cavity phase spectroscopy

The longitudinal modes of a mode-locked bi-directional ring titanium sapphire laser have been frequency stabilized to an Ultra-Low Expansion (ULE) quartz reference cavity in the standard reflection mode topography. An effective rms linewidth of less than 40 kHz with respect to the reference cavity has been measured at the error point of the servo loop. The laser will be used to resolve and lock to doppler-free 2-photon transitions in rubidium vapor. Isolating a three-level lambda type system within the many hyperfine levels of ^87Rb will allow for the study of coherences between the hyperfine levels when the two transitions are driven by the counter-propogating pulse trains from the stabilized bi-directional laser. We will discuss our plans to directly measure the nonlinear dispersion associated with "dark state" resonances in this lambda system using intra-cavity differential phase spectroscopy.

[DP01.29] Characterization of Bi-Pb-Sr-Ca-Cu-O superconductor thin films by Raman scattering

We present in this work the Raman analysis in thin films of the type Bi-Pb-Sr-Ca-Cu-O grown on MgO substrates by spray pyrolisis technique, as a function of the composition and thermal treatment growth parameters. The optical study of these films by Raman scattering shows a serie of vibrational optical modes in the range 350-1000cm-1 at room temperature associated with oxygen atoms on the apical position and related to the superconductor phases. The critical transition temperature in dependence of this frequency is discussed in terms of the oxygen deficiency and the cation stoichiometry in the Bi-O plane.

[DP01.30] Measurements of Hyperfine Structure Intervals in the 3p\ ^4P_J \rightarrow 3s\ ^4P^o_J^\prime Fine Structure Multiplet of ^14N by Diode Laser Spectroscopy

We have measured hyperfine structure intervals in the 3p\ ^4P_J \rightarrow 3s\ ^4P^o_J^\prime multiplet of ^14N using Doppler-free saturated absorption spectroscopy with an external cavity diode laser. Specifically, high-quality spectra of the J=5 øver 2 \rightarrow J^\prime=5 øver 2 (824.4 nm) and J=5 øver 2 \rightarrow J^\prime=3 øver 2 (821.8 nm) transitions were obtained. These spectra were fitted to obtain magnetic dipole (A) and electric quadrupole (B) coupling constants of the 3p\ ^4P_5/2, 3s\ ^4P^o_3/2, and 3s\ ^4P^o_5/2 states. Our values of A and B for these states are more precise than any previously reported measurements of these parameters.(P. Cangiano, M. de Angelis, L. Gianfrani, G. Pesce, and A. Sasso, Phys. Rev. A), 50, 1082 (1994).

[DP01.31] Measurements of Hyperfine Structure Intervals and Isotope Shifts in the 3s\ ^5S_2 \rightarrow 3p\ ^5P_J^\prime Fine Structure Multiplet of Atomic Oxygen by Diode Laser Spectroscopy

We will report on our ongoing experiments on the 3s\ ^5S_2 \rightarrow 3p\ ^5P_J^\prime fine structure multiplet of ^16O, ^17O, and ^18O, which has three transitions near 778 nm. A number of experimental(See, for example, F. Marin, C. Fort, M. Prevedelli, M. Inguscio, G.M. Tino, and J. Bauche, Z. Phys. D), 25, 181 (1993). and theoretical(M.R. Godefroid, G. Van Meulebeke, P. Jönsson, and C. Froese Fischer, Z. Phys. D), 42, 193 (1997). studies of this multiplet have already been reported. However, as yet no complete set of measurements of hyperfine structure coupling constants and isotope shifts exists for this multiplet. We are therefore interested in obtaining such a set of experimental data. Our preliminary studies of the ^5S_2 \rightarrow ^5P_J^\prime transitions in ^16O have revealed Doppler-free spectra with linewidths of 17 MHz, significantly lower than in previously reported work, and we hope to improve this resolution to approach the natural linewidth of 6 MHz. We are also persuing related studies of the 3s\ ^3S_1 \rightarrow 3p\ ^3P_J^\prime multiplet of oxygen, which has three components near 845 nm.

[DP01.32] Semiclassical Representations of Emission, Tunneling or Exchange Rates

Emission, tunneling and exchange rates are related to weighted spectra. A weighted spectrum is defined as D\cal W(E) = \sum_jW_j\delta(E-E_j) where W_j is some defined property of quantum state j. We consider three cases: (1) W_j = f_j^(i) where f_j^(i) is the oscillator-strength for emission from state j into state i; (2) W_j = \Gamma_j, where \Gamma_j is the width associated with tunneling through a barrier; (3) W_j = \Delta E_j, the splitting between a pair of nearly-degenerate states in a symmetric double-well problem (exchange). We consider two types of semiclassical approximations for such rates in regular (nonchaotic) systems. Generalized Bohr-Sommerfeld quantization (Einstein-Brillouin-Keller-Marcus theory) gives formulas for weights W_j for individual states. ``Periodic-Orbit'' formulas give average or large-scale-structure of such rates. We explain both types of formulas, and we show how they are connected to each other. Detailed calculations are carried out for a hydrogen atom in an electric field.

[DP01.33] Aharonov-Bohm Effect and Resonances in the Circular Quantum Billiard With Two Leads

We calculate the conductance through a circular quantum billiard with two leads and a point magnetic flux at the center. The boundary element method is used to solve the Schrödinger equation of the scattering problem, and the Landauer formula is used to calculate the conductance from the transmission coefficients. We use two different shapes of leads, straight and conic, and find that the conductance is affected by lead geometry, the relative positions of the leads and the magnetic flux. The Aharonov-Bohm effect can be seen from shifts and splittings of fluctuations. When the flux is equal to h/2e and the angle between leads is 180^\circ, the conductance tends to be suppressed to zero in the low energy range due to the Aharonov-Bohm effect.

[DP01.34] Computationally Efficient Polarization Potentials for Low-Energy Positron Collisions with Atoms and Molecules

A new suite of codes for efficiently computing the polarization component of the interaction potential for positrons colliding with atoms or molecules has been developed. These codes make use of quantum chemistry techniques to obtain a variational estimate of the distortion energy induced by the electric field of the impinging positron. Our method makes use of a modified adiabatic technique known as the Distributed Positron Model(T.L. Gibson, J. Phys. B) 25, 1321-1336 (1992). (DPM) which approximates short-range correlation effects due to virtual positronium formation and reduces to the correct long-range form of the potential. Within the DPM we have implemented a new positive charge distribution based on a linear combination of gaussians that greatly reduces the amount of computational effort needed to obtain the polarization potential. Scattering results computed with the new distribution will be compared with some of our previous work and new results for e^+--CO_2 \, will be presented.

[DP01.35] Positron Scattering from Atoms and Molecules at Low Energies.

Elastic and inelastic scattering of positrons by atoms and molecules are studied at low energies. The experiments employ a novel technique(C. M. Surko, et al.), this session. using a magnetized beam of cold positrons.(S.J. Gilbert, et al.), Appl. Phys. Letters 70, 1994 (1997); C. Kurz, et al., J. Nucl. Instrum. Methods B 143, 188 (1998). Differential cross section measurements are presented for positron collisions with Ar and Kr in the range of energies from 0.4 to 2.0~eV. These measurements compare well with theoretical predictions. The extension of these measurements to lower energies and to other atomic and molecular targets to measure scattering lengths (e.g., relevant to positron binding) will be discussed. The first measurement of the vibrational excitation of molecules by positrons is presented, studying scattering from CF_4 at energies from 0.2 to 1~eV. Extensions of these measurements will also be discussed.

[DP01.36] New method for studying low-energy positron scattering from atoms and molecules.

We describe a technique under development to study elastic and inelastic positron scattering from atomic and molecular targets(S.J. Gilbert, et al.), this session. in a range of energies previously inaccessible to experiment (0.1~--~5~eV). This technique relies on a source of magnetized, room-temperature positrons stored in a high-efficiency positron accumulator. A cold (0.018 eV) positron beam(S.J. Gilbert, et al.), Appl. Phys. Letters 70, 1994 (1997); C. Kurz, et al., J. Nucl. Instrum. Methods B 143, 188 (1998). is extracted from the accumulator and guided magnetically through a high pressure gas cell (\sim 10^-3~torr), where the positrons scatter from the test gas. We describe a new gas cell and analyzer specifically designed for these measurements. By varying the relative magnetic field values in the scattering and analysis regions, we expect to be able to measure the full DCS when both elastic and inelastic scattering are present.

[DP01.37] Regge Pole Normalization of Relative Electron Differential Cross Sections.

At forward scattering, the recent single Regge pole generalized Lassettre expansion [1] describes a unique curve whose high and low energy limits correspond to optical oscillator strengths (OOSs) for optically allowed and forbidden transitions, respectively. This long-sought after curve normalizes the measured relative electron DCSs to the OOS over the electron impact energy range from near threshold to the first Born approximation limit. A similar curve is also extracted for optically forbidden transitions. Electron-H scattering illustrates the approach, which is general and should inspire immediate experimental verification and a more consorted effort to measure electron DCSs in the difficult to access angular regime near and at zero scattering angles.

\begintabularll \multicolumn1l[1] & Z. Felfli, A. Z. Msezane and D. Bessis, Phys. Rev. Lett. \bf81, \multicolumn1l & 963 (1998) \endtabular

[DP01.38] Resonances in e^- + heavy alkali-metal-atom collisions

Based on the relativistic R-matrix method (U. Thumm and D.W. Norcross, Phys. Rev. A 45, 6349 (1992).) for collisions between slow electrons and Rb or Fr atoms, we provide converged elastic and inelastic scattering cross sections. We include all one-particle relativistic effects and determine valence orbitals in \ell j-dependent effective core potentials which reproduce the lower part of the neutral alkali spectra. The electrostatic interaction between the two outer electrons is modified by a core-polarization correction which is adjusted to reproduce the experimental electron affinity. Partial cross-sections and eigenphase sums are analyzed to determine both energies and widths of negative ion resonances. Our results are compared with recent experimental results for both Rb^- and Fr^- ions (S.J. Buckman and C.W. Clark, Rev. Mod. Phys. 66, 539 (1994).) ^,(A.R. Johnson and P.D. Burrow, Phys. Rev. A 51, 406 (1995).).

[DP01.39] New method for determining the electron forward scattering amplitude

We describe a method for measuring the forward scattering amplitude for electron collisions with atoms or molecules. The scheme uses a gas cell in one arm of an electron interferometer and measures the resulting attenuation and phase shift of the electron matter wave. The complex index of refraction of the gas is determined along with the forward scattering amplitude. Calculations of the scattering of electrons by atoms are performed using a local self-energy potential obtained by treating the atom as an inhomogeneous electron gas. Our results show that the proposed experiments are feasible at high energies.

[DP01.40] High-precision calculations of resonances for three-electron atomic systems

The first fully correlated calculations of resonance for the 1s 2s^2\,^2\!S state of He^- are reported, using the method of complex-coordinate rotation in Hylleraas basis sets. The resonance energy and width are calculated to computational accuracies of 6 ppm and 2% respectively \footnote[1]Y. K. Ho and Z.-C. Yan, Phys.\ Rev.\ A (1998) (submitted).. Comparisons are made with other methods of calculation and with high-resolution experiments. An extension to Li and Li-like ions with similar precision will also be presented.

\vskip 0.5cm

Research support by ITAMP, NSERC, and NSC.

[DP01.41] In Search of Ramsauer Minimum in Elastic Scattering Cross Sections of atoms

Previous investigations on Ramsauer-Townsend minimum show that it occurs (H.P. Saha and Dong Lin, Phys. Rev. A \underline56). 1897 (1998). ^,(H.P. Saha, Phys. Rev. Lett. \underline65), 2003 (1990). in elastic scattering of electrons from Chlorine and Sulfur besides Argon but not from Neon. In search of this minimum we have made an accurate theoretical calculation on elastic scattering of electrons from phosphorus atoms. The electron correlation and polarization effects which are very important in this calculation have been taken into account accurately ab initio using multiconfiguration Hartree-Fock method for bound and continuum wave functions(H.P. Saha, (unpublished).). The scattering length, phase shifts, and differential, total and momentum-transfer cross sections will be presented for electron energies from 0 to 27.2 eV.

[DP01.42] Electron scattering from nitrous oxide

Absolute differential (15 to 130^o) cross-sections for the scattering of electrons from nitrous oxide, N_2O, have been measured over the energy range between 1.0 and 20 eV, using the relative flow technique. Comparisons are made with recent experimental results^1,2 and theoretical calculations, including the R-matrix^2 and Schwinger variational^3 techniques. In general, the agreement with the most recent experiment^1 is excellent. Comparison with theory highlights similar discreprancies to those which have been noted in other triatomic systems. ^1M. Kitajima et al., Private communication. ^2W.M. Johnstone amp; W.R. Newell, J.Phys.B: At.Mol.Opt.Phys. 26, 129 (1993). ^3S.E. Michelin et al., J.Phys.B: At.Mol.Opt.Phys. 29, 2115 (1996). ^4L.A. Morgan et al.,J.Phys.B: At.Mol.Opt.Phys. 30, 4087 (1997).

[DP01.43] Effective Potential for e-Neon and e-Argon Scattering by DCS Minimization

The differential scattering cross-section (DCS) for electrons scattered elastically by neon and argon atoms is studied using a model potential. In the present study the long-range polarization potential is represented by an energy-dependent function and the short-range part is constructed from the non-relativistic Hartree-Fock wave function of the target atoms. The computed DCS obtained using the effective interaction potential for electrons scattered by neon and argon atoms is compared with available published results. In the present study, the parameters contained in the effective potential are determined by the minimization of the DCS with respect to the incident angle and energy. The resulting DCS is found to be in good agreement with avilable experimental and theoretical results in the intermediate energy range.

[DP01.44] Bi-Pb-Sr-Ca-Cu-O/MgO superconducting thin films - Preparation and characterization.

The Bi-Pb-Sr-Ca-Cu-O/MgO thin films were prepared by an aerosol deposition method with aqueous nitrate solutions of 0.02 molarity. For the preparation of samples, experimental designs were applied. In these, the effect of the thermal treatment and that of composition conditions upon formation of superconducting phases were studied. The considered variables were the Bi, Pb, Sr, Ca and Cu content, anneling temperature and time, and heating and cooling time. X-ray diffraction revealed the presence of the Bi2Sr2CaCu2O8+x, Bi1.6Pb0.4Sr2Ca2Cu3Ox, Bi2Sr2Ca2Cu4Ox phases and traces of the Bi2Sr2Ca2Cu3Ox. By SEM we observed mainly forms like plates, rounded and irregular grains. XDS measurements from those forms showed that the grains like plates have the composition of the main phase present in the sample, according with X-ray, and that the rounded and irregular grains are related with phases involving Ca, Cu, Sr, and O. Infrared measurements in the middle and far regions indicated the presence of vibration modes polarized along c axis from the 2212 phases. The Tc of the samples was measured by MAMMAS and the superconducting state was confirmed at low temperature by LTS. The results showed the relationship between the deposition conditions and the film composition with the formation of superconducting phases.

[DP01.45] XAFS Study in Nanocrystalline Fe, Co, Ni and Cu Metallic Powders

X-ray absorption fine structure (XAFS) technique was used to study the local environment of nanocrystalline Fe, C, Ni, and Cu powders between temperatures of 300 and 15 K. From the k edge spectrum measurements, we find that the coordination number in nanocrystalline samples for Fe and Co is 8.6 and 7.4, respectively, which is a little bit larger than that of bulk samples; however, it is reversed for Ni and Cu. This can be explained by the different packing fraction between samples. In general, the bond lengths in both nanocrystalline and bulk metallic powders we studied are roughly the same. The mean-square disorder factor of nanocrystalline samples is always larger than that of the bulk samples, this means that the ordering factor is significantly reduced in nanocrystalline materials. For example, the mean square disorder factor for nanocrystalline Fe, Co and Cu is 0.0064ˇÓ0.0008, 0.0117ˇÓ0.0025, and 0.0083ˇÓ0.0005, respectively; they are slightly larger than the values of bulk samples. From the low temperature studies, we obtained the static (structure) disorder of the samples. In conclusion, we have found that the structure disorder is closely related to the packing fraction of the samples we studied.

[DP01.46] Fast Switching of a 16 W Diode Laser

To achieve a fast switching of a 16W pigtailed diode laser, we built a micro-structure which mechanically changing the alignment of two fibers. A piezo film is used to generate the fast movement.

[DP01.47] Experimental Observations of Passive Cooling in a Penning Trap

H and He ions have been passively cooled using a Penning trap which has been optimized for transverse resistive cooling. The cooling time constants measured agree with theoretical predictions and vary depending on the ion species, the cooling duty cycle and the Q of the tuned circuit. The tuned circuit is formed by connecting opposite quadrants of the split hyperboloidal ring with superconducting Nb/Ti inductors placed in a 4.2K liquid helium bath. The resulting resonant circuit (Q=1800 at 7.4 MHz) is then used to cool the ions by varying the trap bias voltage so as to allow the f^+ ion motion to overlap the resonant frequency of a tuned circuit. Ion signals are magnetically coupled to a wide-band cryogenic GaAs FET pre-amplifier, narrow-band amplified and diode detection system. Some additional features of the ion spectra, such as response to heating by f^+ - f^- and growth/decay of signal peak components, will also be discussed.

[DP01.48] Injection Locking of Fiber-Coupled Laser Diode Arrays

We report injection locking of high-power, fiber-coupled laser diode arrays (LDAs). These LDAs are commonly used for optical pumping on the Rb D1 line at 795 nm in order to spin-exchange polarize ^3He and ^129Xe gas. Free-running (without injection locking), the nominal LDA spectral width is nearly 2 THz, while the pressure-broadened Rb resonance is typically less than 100 GHz wide. With injection down an LDA fiber of narrow spectrum light from a lower-power master laser, more than 70% of the output power from an LDA is collapsed into a line less than 20 GHz wide, corresponding to an amplification of master laser light of about five. We have investigated the dependence of injection locking amplification on master laser frequency and power, and examined limitations and applications of this technique.

[DP01.49] Physical Characterization and Computer Modeling of a Laser-Powered Atomic Nitrogen Source

Molecular nitrogen dissociates if illuminated with a pulsed CO_2 laser.(G.C. Caledonia, R.H. Krech, B.D. Green, and A.N. Pirrl, U.S. Patent No. 4,894,511 (16 Jan 1990).) As ionized nitrogen absorbs laser energy through inverse Bremstrahluung, the temperature of the nitrogen increases, allowing greater dissociation and ionization as a shock wave is formed. We report ratios of dissociated molecules and of ionized species for such an atomic nitrogen source under ultrahigh-vacuum conditions as a function of power imparted by a pulsed CO_2 laser (pulse down to 20 \mus with energy up to 10 J), of the time between gas pulse and laser pulse, of the N_2 pressure, and of the gas nozzle shape. We model this nitrogen system using a quasi-one dimensional, non-steady-flow model that incorporates a shock-capturing algorithm(N.H. Kemp, AIAA Paper 84-1569 (1984).) to yield these ratios and the expected atomic-beam energy. We demonstrate the suitability of this technique for growing nitride thin films.

[DP01.50] Low-energy Positron-molecule Scattering: Cross sections and Annihilation probabilities.

Studies on low-energy positron--molecule scattering have gained increasing interest in last years. Our group has recently implemented a version of the Schwinger Multichannel Method for positron--molecule scattering [J.S.E. Germano and M.A.P. Lima, Phys. Rev. A 47, 3976 (1993)] and developed a technique for using its wave function to calculate the annihilation parameter, Z_eff [E.P. da Silva, J.S.E. Germano and M.A.P. Lima, Phys. Rev. A 49, R1527 (1994)]. This parameter, along with integral, differential and momentum transfer cross sections, has permited us to investigate some important aspects of positron--molecule collisions, such as the importance of some specific excited states of the target in the collision and annihilation processes; how the target affects the collision period; the existence or not of resonant processes and other interesting details. We have reported successfull aplications of our method to He atom and H_2 [E.P. da Silva et al. (1994), cited above] and also to C_2H_4 [E.P. da Silva, J. S. E. Germano and M. A. P. Lima, Phys. Rev. Let. 77, 1028 (1996)]. In this conference, we will present results for two isoelectronic molecular systems, C_2H_2 and N_2. We will also present preliminary results, for He and H_2, of annihilation probability maps, which tell us where annihilation is more likely to take place in the target's field.

[DP01.51] Scattering Resonances in the Extreme Quantum Limit

This poster will present a number of interesting scattering resonance phenomena of systems when the scattering particle wavelength is much longer than all other scales in the system. Proximity resonances, which are related to Dicke sub-luminescence, occur when two or more small s-wave resonant scatterers are placed such that their on-resonance cross sections overlap. Experimental evidence of proximity resonances has recently been found in a microwave scattering experiment.

Further, we consider the problem of a particle moving in the field of two attractive, large scattering length potentials. A p-wave threshold resonance appears in the scattering spectrum of the system when the potentials are one scattering length apart. This threshold resonance is connected with the release of the antisymmetric bound state of the double well system into the continuum. Further, we find that the total cross section of the two potentials vanishes as their separation becomes much less than the single well scattering length. This result has application to the problem of scattering in the presence of an external confining potential. The Efimov and Thomas effects are discussed in the context of the double well system.

[DP01.52] Generalized oscillator strength for argon

\ \ \ Using the random phase approximation with exchange (RPAE), we find that the influence of correlation and exchange effects on the position of the characteristic minimum in the generalized oscillator strength of Ar is insignificant. Also, our first Born approximation predicts the position of the minimum accurately provided that accurate target wave functions are employed. Our results agree excellently with measurements and are expected to be applicable equally to the corresponding subshells of Kr and Xe.

[DP01.53] Elastic scattering of slow positrons by atoms

A generalization of the RPAE method proposed earlier in [1] for positron-He scattering, is used here to describe the elastic scattering of slow positrons from some atoms (Li, Be, Ar, Kr, Xe). In this method as a first step the positron wave functions are calculated in the field of a "frozen" Hartree-Fock core of the atomic ground state. After that the corrections to the positron phase due to the correlational interaction is found through the self-energy part which is calculated in the lowest (second) order of perturbation theory in the Coulomb matrix elements. For slow positrons it is important to take into account the possibility of virtual positronium formation. The straightforward consideration of this process is very complicated, therefore we restrict ourselves by a simple method proposed in [1]. Namely, we introduce the correction for the positron binding energy in the energy denominator by replacing the energy of the hole Ei by the difference Ei-Ep where Ep is the positronium binding energy (Ep=-0.5Ryd). Results of calculations performed by this method are in a reasonably good agreement with experimental data where they are available. 1. Amusia M.Ya. et al. J. Phys. B. 9, L531 (1976)

[DP01.54] Particle Capture by Triple Collisions

It has long been known that the predominant mechanism in the capture of an electron from an atom in a collision with an energetic ion proceeds through a double collision mechanism first described by L. H. Thomas [1]. This gives rise to the ``Thomas Peak'' in the differential cross section, which has now been observed [2]. In turn, the Thomas process leads to the dominance of the second Born term over all other terms in the Born series [3]. The Thomas capture process can be described classically as a purely kinematic phenomenon [4]. Examination of the kinematics shows that for certain mass ratios, the double collision process is kinematically forbidden [4]. Recently, we have shown that for some of these mass ratios, it is nevertheless kinematically possible to capture by triple collisions. The question, which we are now studying, is how these processes affect the high energy behavior of the terms in the Born series.

// [1] L.H. Thomas, Proc. Roy. Soc. A114, 561(1927). For a recent review see J. H. McGuire, et al., in Atomic, Molecular and Optical Physics Handbook, Ed. by G.W.F. Drake (AIP Press, 1996). // [2] E. Horsdahl-Pederson, et al. Phys Rev. Lett. 50,1910 (1983). // [3] K. Dettmann and G. Leibfried, Z. Physik 218, 1 (1969). // [4] M. Lieber (unpublished, but see the article by McGuire, et al. cited in Ref. [1]).

[DP01.55] Positron Collisions with Lithium Atoms at Intermediate Energies Using Time-Dependent Close-Coupling Methods

Time-dependent close-coupling methods are used to calculate transfer ionization cross sections for positrons incident on a lithium target at 20, 30, and 40eV. The initial positron-lithium wave function is constructed from a positron wave packet and a pseudo-orbital for the ground state of lithium. This wave function is then propagated time-dependently by explicitly solving a system of coupled partial differential equations in the potential field of a one-body l-dependent pseudo-potential and the two-body Coulomb potential. Close-coupling results are obtained for total angular momenta of L = 0 - 8. For L>8, lowest-order many-body theory is used to calculate both the ionization and positronium formation partial cross sections. Comparisons are only made with other theories as no experimental results are presently available.

[DP01.56] Chirped microwave multiphoton transitions

We have observed 3, 4, and 5 photon microwave transitions in Rydberg states of potassium using chirped pulses of microwave radiation. Pulses of microwaves with frequencies from 350 to 450 MHz and 700 to 800 MHz with frequency chirp rates from 0.1 MHz/ns to 10 MHz/ns. The pulses have peak fields of up to 10 V/cm, corresponding to Rabi rates of 0 to 100 MHz. These pulses are incident on ns Rydberg atoms of K in a static electric field within a microwave transmission line. We present measurements of population transfer to higher lying states as a function of intensity, chirp rate and sign of chirp. The data is compared with both a Landau-Zener models and numerical integration of Schrödinger's equation.

[DP01.57] Use of Pulsed Electric Fields to Manipulate Atomic Wavefunctions.

The manipulation of Rydberg atom wavefunctions by well characterized electric field pulses is investigated. Rubidium np Rydberg atoms with n = 390 are subject to a rapidly applied "DC" electric field pulse whose duration (\sim 100 - 200ns) is long compared to the classical electron orbital period (\sim 9 ns). Following application of this pulse, the final state of the atom is examined using a short (\sim 2 ns) half-cycle probe pulse. The data are analyzed with the aid of the results of classical trajectory Monte Carlo (CTMC) simulations. The present results show that, because Stark precession during the "DC" pulse leads to a cyclical evolution of the angular momentum, the final state of the atom can be controlled by varying the width of the "DC" pulse. If the duration of the pulse is around a fourth of the Stark precession period, a nearly circular state is thereby created. Further work is underway to fully explore the potential of this technique.

[DP01.58] Use of Rydberg Atoms to Probe Electron Atachment to Molecules

Rydberg atoms provide a unique source of low-energy essentially-free electrons for use in studies of electron attachment reactions. Dissociative and non-dissociative electron attachment is being investigated by measuring the velocity and angular distributions of positive and negative ions produced through electron transfer in collisions with velocity-selected K(np) Rydberg atoms. Because of post-attachment interactions between the product ion pairs, these distributions reflect both the lifetimes and the decay mechanisms of excited intermediates initially formed by electron capture. The data are analyzed using a Monte Carlo collision code that models the detailed kinematics of the reactions. Measurements of the lifetimes of the intermediates formed in non-dissociative electron attachment to C_7F_14 and C_6F_6 are underway, together with studies of temperature dependences in the dissociative electron capture by CCl_3Br.

[DP01.59] Study of surface hollow atoms decaying outside thin C foils

The formation of hollow atoms by passing Ar^17+ ions through thin C foils has been studied at energies in the range 1-4 MeV/q at the 88-inch Cyclotron of the Lawrence Berkeley National Laboratory. Surface hollow atoms were observed even at relatively high energy. The decay of these surface hollow atoms has been observed for the first time downstream from very thin (6 atomic layers) C foils comparing the X-ray spectra emitted inside thick and very thin foils. This result may be explained as due to the capture of C K electrons (whose velocity is still large compared to that of the ions) into the M shell of the ion during the evolution of the transient quasi-molecule formed by the close encounter between the ion and a target C atom.

[DP01.60] Core-Scattered Combination Orbits in the M = 0 Rydberg-Stark Spectrum of Helium

Laser-fast-atom-beam scaled-energy spectroscopy has been used to measure M = 0 Rydberg-Stark recurrence spectrum of the two spin forms of helium. A 4 keV beam of collisionally prepared metastable helium atoms is excited by tunable collinear UV laser photons to n states between 15 and 30 under constant scaled energy conditions, \epsilon = EF^-1/2, where E is the excitation energy and F is the electric field. Spectra are taken for 61 different scaled energies ranging from -2 to -3.5. When the recurrence spectra are viewed globally as a function of scaled energy and scaled action, regions of clustered core-scattered combination recurrences are revealed, highlighting the importance of combination scattered orbits in the Stark spectrum. Data are interpreted using closed orbit theory. Integrating combination orbit recurrence peaks over scaled-energy allows direct comparison between singlet and triplet core scattering. Agreement is found with e + He^+ scattering using quantum defects, demonstrating the link between electron-ion scattering and Rydberg spectra.

[DP01.61] The Structure of the Stark Recurrence Spectrum

The primary goal of research in "quantum chaos" is to explore the extent to which the methods and natural intuition of classical mechanics can be used to elucidate the complex and often surprising behavior of large quantum systems. Much recent work is based on the deep connections between classical periodic (or closed) orbits and the quantum spectrum revealed by semi-classical "trace formulas". One important application is the analysis of the "recurrence spectrum" of Rydberg Atoms in strong static electric fields in which peaks in the measured "recurrence spectrum" are associated with individual classical periodic orbits. Here we provide a purely quantum mechanical explanation for peaks in the recurrence spectrum based on the regularities of the Stark photoabsorption spectrum which serves to demystify these (still complex) representations of the quantum spectrum and provides new insight into the functional differences between hydrogen and helium Rydberg atoms in strong fields that reemphasizes the remarkable correpondence between the classical and quantum theory for these systems.

[DP01.62] Classical evolution of quantum elliptic states

The hydrogen atom in weak external fields is a very accurate model for the multiphoton excitation of ultrastable high angular momentum Rydberg states, a process which classical mechanics describes with astonishing precision. We show that the simplest treatment of the intramanifold dynamics of a hydrogenic electron in external fields is based on the elliptic states of the hydrogen atom, i.e., the coherent states of SO(4), which is the dynamical symmetry group of the Kepler problem. Moreover, we also show that classical perturbation theory yields the exact evolution in time of these quantum states, and so we explain the surprising match between purely classical perturbative calculations and experiments. Finally, as a first application, we propose a fast method for the excitation of circular states; these are ultrastable hydrogenic eigenstates which have maximum total angular momentum and also maximum projection of the angular momentum along a fixed direction.

[DP01.63] Coherent stabilization of zero-electron-kinetic-energy (ZEKE) states

The accuracy of zero-electron-kinetic-energy (``ZEKE") photoelectron spectroscopy rests on the ultralong lifetimes of the high-n, high-\ell Rydberg states that are responsible for the ZEKE signal. However, a few-photon process cannot excite electrons directly from the low-\ell ground state to the high-\ell ZEKE manifold. In this paper we show that using the dynamics of Rydberg Stark states in time dependent external fields it is possible to control coherently the angular momentum of Rydberg electrons, and therefore also their lifetime. We derive explicitly two different schemes, based on simple, short electric dc pulses, which populate precisely those high-\ell, long-lived Rydberg states that are necessary for accurate ZEKE experiments. The high-\ell states that we construct are also Stark eigenstates, therefore a moderate dc external field can eventually enforce cylindrical symmetry and lock the ZEKE electrons in the stable, long-lived high-\ell manifold.

[DP01.64] Extending closed-orbit theory using quantum defect methods

Closed-orbit theory(M.L.~Du and J.B.~Delos, Phys. Rev. A 38), 1896 (1988). has been successful in describing the near-threshold photoabsorption spectrum of hydrogen-like atoms in strong external fields. However, the usual form of closed-orbit theory is not well suited for describing above-threshold physics or complex multichannel cores. We propose a generalization of closed-orbit theory which naturally includes these features. By establishing an explicit connection between multichannel quantum defect theory(D.A.~Harmin, Phys. Rev. A 26), 2656 (1982). (MQDT), closed-orbit theory, and semiclassical S-matrix theory(E.B.~Bogolmony, Nonlinearity 5), 805 (1992)., the scattering of the Rydberg electron by the core and by the long range field is treated on equal footing. Our formulation retains MQDT's ability to describe multichannel, above-threshold physics, and closed-orbit theory's description of energy-smoothed or time-domain spectra in terms of classical actions.

[DP01.65] Rydberg State Spectroscopy in Highly Charged Ions

We have measured the ultraviolet (UV) spectra of Rydberg state transitions excited in a fast beam of highly-charged ions produced at the Notre Dame tandem accelerator laboratory. By coupling a cooled CCD detector to a vacuum-UV spectrometer, we have expa nded our program of atomic physics studies of highly-charged ions using position sensitive photon detection at low light levels. We have detected transitions involving Rydberg states as high as principal quantum number n=19 in ionization stages up to +20 . For the higher angular momentum components of Rydberg states, the binding energies are sensitive to atomic core polarization perturbations arising from long range electron-ion interactions. We will present measurements of core polarization effects on Rydberg transition energies in highly charged sulfur and nickel ions.

[DP01.66] Energy levels of CIV: The polarization method

We previously published [Can. J. Phys. 75, 11 (1997)] a calculation of the generalized polarizabilities up to multipole order 3, as well as certain higher-order hyperpolarizabilities, for two- electron atoms and ions of Z=2 through 6 and 10. Now we apply some of these results to calculate excited-state energies in three times ionized (lithium-like) carbon. For states with angular momentum L greater than 2 accurate results are obtained using an asymptotic polarizability expansion that includes nonadiabatic effects. Relativistic and retardation effects are also included. Comparison is made with recent measurements, and a critical discussion of the correct form of the expansion is given. For completeness we will also present similar results for lithium-like neon and oxygen ions.

[DP01.67] Angle-of-Approach Studies of Electron Capture from Linear Stark States

We studied electron capture by singly charged ions from linear Stark Rydberg states of Na as a function of the angle of impact and of projectile velocity. The target, the topmost state of the n = 24 Stark manifold (24top), is prepared by a two-step laser excitation in an electric field F_Stark= 160 V/cm. Using the ``Stark Barrel''(J. L. Horn, D. M. Homan, C. Hwang, W. L. Fuqua III and K. B. MacAdam, Rev. Sci. Instrum. 69), 4086 (1998). the target is aligned by adiabatically switching the field to a preset low value and a new direction in the plane of the ion and Na atomic beams. A Li ion source was operated at accelerating voltages 300-2000 V to cover the reduced velocity range v = 1.0 - 2.5. To correct the angular dependence for systematic effects, we normalized the 24top capture cross section with that from the spherically symmetric 25s_1/2. This complements an earlier study by Homan et al.\footnote D. M. Homan, O. P. Makarov, O. P. Sorokina, K. B. MacAdam, M. F. V. Lundsgaard, C. D. Lin and N. Toshima, Phys. Rev. A 58, 4565 (1998). at lower v.

[DP01.68] Electron Capture from Aligned Rydberg p States

We have measured, for the first time, the cross section for electron capture by singly charged Li ions from the 25p Rydberg state of Na, as a function of the angle between the incoming ions and the symmetry axis of the state. The target is prepared by a two-step laser excitation, via the 3p_3/2 as an intermediate, in a electric field of about 25 V/cm to break symmetry and allow the p-to-p transition. The interaction between the field-induced dipole moment of this state and the field itself can then be used to ``turn'' and spatially align the atomic system. After excitation, using the ``Stark Barrel''(J. L. Horn, D. M. Homan, C. Hwang, W. L. Fuqua III and K. B. MacAdam, Rev. Sci. Instrum. 69), 4086 (1998)., we align the p state by adiabatically reducing the electric field to a low value in a new direction in the plane formed by the ion and Na atomic beams. The Li ion projectiles from a thermionic emission source were accelerated to a reduced velocity v = 1.2. Preliminary results show the expected reflection symmetry of the p state.

[DP01.69] Preliminary Results for the Interaction of CO with Iron Surfaces

The interaction between CO and iron is of interest to both the industrial and military communities. We are using DFT methods to probe this interaction as an initial step to understanding the carburization of iron. Preliminary structural and energetic data will be presented for CO interacting with the (111) and (100) surfaces of BCC iron.

[DP01.70] The effect of correlations on the \alpha-\gamma transition in Ce

In the present work the attempt is made to include correlation effects due to strong on-site f-electron repulsion in the first-principled electronic structure calculations. The parameters of Anderson model and parameters of the f-f hopping are calculated self-consistently. The effect of strong correlations is included via the construction of the density functional with the Gutzwiller wave function. The effect of these correlations on the \alpha-\gamma transition in Ce is discussed.

[DP01.71] Moments Expansions for the Correlation Energy of an Exactly Solvable Problem

In this work we study the ground-state properties of the model Hamiltonian \[H=\frac12\sum^N_i=1\left( -\fracddx_i^2+ømega^2x_i^2\right)+ \sum^N_i,jg^2x_ij^2\] which represents a set of N one-dimensional correlated harmonic oscillators. Here the parameter g describes the coupling between oscillators and may be either real (attractive case) or purely imaginary (repulsive case). We wish to study the correlation energy of the system by two methods: the Lanczos tridiagonal scheme and a newly developed moments expansion AMX (alternate moments expansion). Comparisons will be made with other moments expansions, in particular the CMX-LT(2) and CMX-LT(3) series. Furthermore the Lanczos method is used to evaluate the singlet-triplet energy gap.

[DP01.72] Ground State of a Two-Level System with Phonon Coupling

The ground state of a two-level system coupled to a dispersionless phonon bath is studied using both a connected moments expansion and a truncated Lanczos tridiagonal scheme. We consider the spin-Boson Hamiltonian \[H=-\delta_0\sigma_x+\sum_k\hbar ømega_k a^_ka_k+\sum_kg_k (a^\dagger_k+a_k)\sigma_z \] where \delta_0 is the bare tunneling matrix element and g_k represents the coupling to the k phonon modes. Such systems have found relevance in applications to molecular polaron formation, exciton motion and attenuation of sound in glasses. Our results are then compared to those of variational methods as well as an exact numerical diagonalization.

[DP01.73] The Fermi gas at finite temperature: Why do we need the anomalous diagrams?

At finite temperature, a Fermi gas can have states that hold simultaneously particles and holes. Hence the so called anomalous diagrams are nonzero and do contribute to the thermodynamic potential. We calculate this contribution at low temperature and find their effect on the specific heat within the RPA approximation. We show that in this case a new infinite set of diagrams must be included. They contribute a TLnT term that cancels a similar term that appears due to exchange. We conclude that this particular logarithmic behavior is not related to the T/LnT that shows up in zero temperature treatments of the specific heat when screening is neglected.

[DP01.74] Positron annihilation in sandstone and carbonate rocks

A broad variety of well-characterized sandstone and carbonate (limestones, dolomites, marbles) rock samples have been investigated by use of the positron annihilation methods (Doppler Broadening and Lifetime Spectroscopy) and the Scanning Electron Microscope. Rocks have been examined in different states (dry or soaked in water or hydrocarbons) and during heating/cooling cycle (from room temperature to 220 ^0C). The S-parameter and the mean lifetime values depend mainly on the composition of the rock, whether sandstone or carbonate, and its physical state, whether dry or soaked, and to a lesser degree on the structure of the rock. The dependence of the S-parameter on the temperature over the range studied is small and monotonic. We propose a qualitative physical model of the positron annihilation process based on the compositional and structural differences among the rocks. The sensitivity of the positron annihilation signal, particularly the S-parameter, to the composition and structure of sandstone and carbonate rocks, and the small dependence on temperature, suggest that a new type of nuclear well-log probe could be developed based on the positron annihilation process.

[DP01.75] Electron-optical-phonon interaction Hamiltonian for wurtzite superlattices

We present the Fröhlich-like carrier--optical-phonon interaction Hamiltonians for wurtzite superlattices based on the macroscopic dielectric continuum model and the uniaxial model. In addition to confined and interface optical phonon modes, propagating modes may exist in wurtzite superlattices due to the anisotropy in the phonon dispersion. This happens when the phonon-dispersion curves of two materials overlap. The confined modes have tails and contribute to the carrier scattering in the other material.

[DP01.76] Migration of oxygen-vacancy defects in silica: First-principles calculations

First-principles calculations are used to study the migration of oxygen-vacancy defects in silica. We calculate the minimum energy path and activation energy barrier of migration using the nudged elastic band method. Both "planar" and "puckered" structures of positively charged and neutral oxygen-vacancy defects are considered.

[DP01.77] NRM, EPR and TSD: alternative methods of determining paleoenviromental changes in a lithological contact.

Rock magnetic, paramagnetic and dielectric studies have been conducted in sedimentary rocks close to a lithological contact in Cretaceous northeastern Venezuela. Anomalies of stable natural remanent magnetizations (NRMs), at the vicinity of the formational transition, reveal the presence of secondary magnetite and pyrrhotite (both minerals stable in reducing conditions). Electron Paramagnetic Resonance (EPR) experiments also show a signal associated with organic material, indicating possible reducing conditions. On the other hand, rock magnetic (Lowrie tests) experiments suggest the presence of hematite at the vicinity of the contact, the likely by-product of oxidation. In fact, Thermally Stimulated Depolarization (TSD) and EPR experiments indicate the existence of water molecules, bounded to different sites in the material, whose abundance appears to increase towards the contact itself and seems to be associated with the hydrotermal alteration focus. Thus, the integrated physical evidence suggests that this lithological contact has acted, in the past, as a focus of alternated reducing and oxidizing conditions that have affected the adjacent strata and echoed paleoenvironmental changes in the sedimentary basin

[DP01.78] Self-Affine Analysis on Curved Reference Surfaces: Self-Affine Fractal Characterization of a TNT Fracture Surface

A trinitrotoluene (TNT) fracture surface image is characterized in terms of a self-affine fractal structure. The fracture surface was produced by high acceleration in an ultracentrifuge when the TNT strength was exceeded. An atomic force microscope (AFM) captured the topography of a 4 micron square region on the fracture surface. The present analysis supports a self-affine fractal description of the TNT fracture surface (wavelengths of 0.016 micron to 4.0 micron) and provides a new perspective on fracture processes in TNT. An essential step in self-affine fractal characterization of surfaces is the determination of reference surfaces. A self-affine fracture surface can be described in terms of a single-valued height function. In the TNT fracture surface, single-valued height functions, which describe surface texture, can only be defined with respect to curved reference surfaces. By employing curved reference surfaces, we have demonstrated that self-affine fractal scaling can be used to characterize the TNT fracture surface. This provides important information that is not evident in the analysis of individual surface scans.

[DP01.79] Rapid-Converging Linear Models for Carbon Energetics on Shock and Static Compression

The series-expanded functional forms have been widely used for describing condensed matter thermodynamics at high pressure. For many shock-loaded matters in a single phase, shock velocity shows a rapid-converging linear behavior when expanded with particle velocity. For static compression, there are a couple of well-known, series-type isotherm models in good agreement with experiments regardless of the convergence of the series. An appropriate expansion variable is needed in practice, however, to assure a rapid convergence of the series in truncated form. We choose a generalized finite strain function for cohesive energy model and determine the expansion variable that makes the model linear. Then we derive the Gruneisen function from the two linear models and compare it with the relevant data especially for graphite and diamond phases of carbon. We also show that the linear isotherm model describes the experimental curves well for various solids.

[DP01.80] Epitaxial Insulator Layers on Si by MBE

During previous years Si-MBE has become an attractive tool for the realization of high quality epitaxial layers, new thin film structures and electronic devices, which are difficult, if not impossible, to obtain by use of the more classical growth techniques. Heteroepitaxial growth of dielectric films on Si substrates is of considerable interest in the formation of SOI-structures and 3D-integrated circuits for microelectronics applications. CaF_2 crystallizes in the cubic fluorite crystal structure which is closely related to the diamond structure of Si. The lattice mismatch is only 0.6% at room temperature. In the course of the present work CaF_2 epitaxial films were grown on Si(111) substrates by means of MBE. The Si substrates were chemically cleaned, and the final volatile oxide was desorbed in situ by heating to 850 ^oC. CaF_2 was evaporated from a Knudsen-type cell by use of a graphite crucible, while the growth temperature was held at 650...750 ^oC. RHEED has been used to monitor the film growth in situ and to study the epitaxial quality. Further, RBS has been applied to look at defects and to measure strain in the CaF_2 layer. Usually good crystallographic prop erties are achieved under optimum growth conditions, with values of \chi _min < 5%.

[DP01.81] Measurements of Electrostrictive Effects in Low Permittivity Dielectrics

Electrostrictive deformation of a dielectric material caused by an electric field may be undesirable in devices where fixed geometries are required or components are susceptible to fatigue. Selecting suitable materials for particular applications requires knowledge of the relationship between electrostriction and other material properties. Most of the properties of packaging materials have been well established, but reliable data on the electrostriction coefficients of low permittivity oxides and nitrides used in these multilayer packages are currently unavailable. Measuring electrostrictive effects in materials requires extremely sensitive instrumentation. In this work, a modified single beam interferometer capable of sub-angstrom resolution in displacement is presented, along with a newly modified dynamic compressometer for resolving fractional changes in capacitance of the order of 10^-6. The interferometer is a modified Michelson-Morley instrument with modifications made to detect changes in interference fringe intensity for very small changes in path length. For the compressometer, a high sensitivity (10^-15 F) GenRad 1615 bridge is coupled with a lock-in amplifier to detect the change in capacitance caused by cyclic uniaxial stresses on samples. The measurements confirmed by both methods are used to establish a set of reliable and accurate data of electrostriction coefficients for low-dielectric constant materials. This study is supported through the National Science Foundation grant DMR-9634101.

[DP01.82] Investigation of Interaction of Helium with Solid Hydrogen

The interaction of ^3He^+ ion with hydrogen molecule and solid hydrogen have been studied using the Hartree-Fock-Roothaan procedure, a cluster involving the helium ion inside an assembly of six hydrogen moleules being used to simulate the interaction of the ion with solid hydrogen. The binding of the ion with the surrounding molecules is found to be sensitive to the influence of many-body effects and is found to be stronger than He^+ bound to hydrogen molecule, indicating stronger binding in the solid than in the gas. Details of our results and also of our study of charge exchange processes involving possible conversion of helium ion to helium atom will be discussed.

[DP01.83] Investigation of Stark Energy Levels of 4I9/2 and 4I11/2 Manifolds and Effects of Temperature on a Sharp Emission Line of Nd3+ in YVO4

The Stark components of the 4I9/2 and 4I11/2 manifolds have been analyzed using the room temperature fluorescence spectra for the 4F3/2 * 4I9/2 and 4F3/2 * 4I11/2 transitions of Nd3+ in YVO4 laser host. The thermal effects on the linewidth, line position, and line shift of the strongest emission line (1061.7 nm) due to the inter-Stark transition R1 * Y1 within the 4F3/2 * 4I11/2 manifold transition have been also investigated. The linewidth of this transition has been found to increase with increasing temperature and the line shifted toward the longer wavelength. The experimental results of the temperature-dependent width and shift of this line are explained by the existing phonon-ion interaction theory. *Supported by the NSF Grant No. DMR-9616608.

[DP01.84] X-ray fluorescent spectra and electronic structure of KNbO3 and KTaO3

Results of experimental and theoretical studies of the electronic structure of KNbO3 and KTaO3 are presented. Soft x-ray emission spectra (XES) were excited near Nb 3d threshold by of monchromatic synchrotron radiation. Our experiments were performed at the ALS, Lawrence Berkeley Lab. It is found that Nb M4,5 XES are strongly dispersing with the excitation energy which is due to selective excitation of Nb 3d electrons to the 5p and 4f levels. At an excitation energy of 221.1 eV resonant Coster-Kronig transitions (M5 to M4) are observed. We also measured for the first time Nb M4 and Ta N4 emission of KNbO3 and KTaO3. The density of states of the Nb 4d and Ta 5d valence bands of these compounds is presented. The experimental results are compared with theoretical calculations of the x-ray emission spectra.

[DP01.85] Electronic structure of cadmium tungstate (CdWO_4).

In previous work, we studied the electronic structure of several tungstate crystals having the scheelite structure.(Y. Zhang \emet al.), Phys. Rev. B \bf57, 12738 (1998). The wolframite structure is another common form for tungstate materials, having the space group P2/c with two tungstate units per primitive unit cell.(A. W. Sleight, Acta Cryst. B \bf28), 2899 (1972). Within the framework of density functional theory, using the same computational tools as in our previous work, we have studied the electronic structure of the wolframite material CdWO_4,(J. Macavei and H. Schulz, Z. Krist. \bf207), 193 (1995). which is used in X-ray detectors. We find the valence band width to be 7~eV with O 2p character at the top of the valence band and a sharp Cd 4d responance at the bottom of the valence band. The top of the conduction band has mainly W 5d character.

[DP01.86] Surface magnetoplasmons of corrugated films

Effects of applied magnetic fields B_0 on surface magnetoplasmon polaritons of surface corrugated films in contact with semiinfinite superlattices are investigated. The B_0 fields are assumed to be along the growth axis of the multifilm arrays (perpendicular configuration). The combination of the Rayleigh-Fano and transfer matrix approaches are applied to calculate the amplitudes of specular reflection and diffused scattered waves for light incident with p-polarization. Frequencies of the specular reflection minima are used to obtain the surface magnetoplasmon polariton dispersion relations, with the results exhibiting the dependence on the surface corrugation and the strength of the applied magnetic fields.

[DP01.87] Nanometer-scale Si selective growth on Ga-adsorbed voids in ultrathin SiO_2 film

We examined nanometer-scale Ga selective doping by Si growth on Ga-adsorbed void surfaces in ultrathin SiO_2 film on Si(111) surfaces using STM. Voids with the size of 5-20 nm were formed by thermal decomposition of 0.3-nm-thick SiO_2 film. The voids were plugged with stable \surd3x\surd3-Ga structure by Ga deposition and subsequent annealing. Then the selective growth was performed by introducing disilane gas. Si crystals were selectively grown only on the void surface at 460-550 ^oC. The growth started from the edge, and the islands grew in a layer-by-layer fashion. This is the same mode as that on the void without Ga, but the growth rate on Ga-adsorbed void was decreased. After the sample was annealed, the grown surface showed \surd3x\surd3 structure again. It is thought that incorporated Ga atoms in the Si crystal partly segregated during the selective growth and they reconstructed on the top surface. These results show that Ga-doping dots of nanometer-scale can be formed by selective epitaxial growth using ultrathin SiO_2 mask. This work was supported by NEDO.

[DP01.88] STM Observations of dislocations in graphite

A wide variety of dislocations observed by STM will be discussed in this paper. Dislocations observed include single atom lines to several thousand atoms wide bands. Some of the bands had abc structure while others contained "giant atoms" indicating relative rotation between layers of graphite. In one case strong electronic modulations are observed on one side of the dislocation line. Some of the properties of these lines such as the movement of the lines with scanning, intensity variation with bias voltage, and interaction between the lines will also be presented.

[DP01.89] Polyhedral Oligomeric Silesquioxane (POSS) Nano-Reinforced Polyurethane (PU) Thin Films

Polyurethane is an elastomeric material containing solid-like hard segmented and rubbery soft segmented sequences. The Air Force Research Laboratory (1,2) developed a procedure to synthesis a new hybrid polymeric (PU)/ inorganic (POSS) material in which the POSS reinforcement ((RSiO3/2)n (n=8)) is covalently attached to the hard segments of the chain strengthening the sequence and inducing ceramic like properties (heat resistance, oxidation resistance, mechanical strength, etc.) to the polymeric material. Conventional thin film forming technology (Langmuir-Blodgett, spin casting, solvent casting, etc.) was employed to prepare thin films (>1000 angstroms) of PU, PU/21Electron Microscopic (TEM) examination of Langmuir films of increasing POSS percentage transferred to silicon monoxide substrates indicate a transition from an amorphous island like (dewet) surface morphology for PU/0uniform film containing ordered sheets of crystalline POSS particulates (~25nm in size) at higher POSS percentages. The POSS nano-reinforcements tended to impart an amphiphilic nature to the polymeric/ceramic hybrid film (POSS at water surface PU at free surface). Atomic Force Microscopy (AFM) studies indicated similar morphological conclusions and provide qualitative tribological information. Variable angle Xray Photonelectron Spectroscopy (XPS), Selected Area Diffraction (TEM) and High Resolution TEM results will be shown to predict the structure and orientation of the POSS reinforcements. 1. Lichtenhan, J.D., Comments Inorg. Chem. 17, 115-130 (1995). 2. Schwab, J.J., Lichtenhan, J.D., et. al., PMSE Preprint, 77, 549 (1977).

[DP01.90] Characterization of Surface Roughness and Height Fluctuations of Microcrystalline and Amorphous Silicon Thin Films

Surface morphology of silicon thin films plays an active role in many optoelectronic device applications. In particular, for improving the performance of thin film Si solar cells, a quantitative physical understanding of Si surface morphology in terms of its surface roughness and correlation function of height fluctuations is needed. In the present work, various types of Si surface including those of amorphous films, microcrystalline films grown with different grain sizes, n- and p-type bulk crystals with surface roughness purposedly generated by chemical etching have been studied by using the techniques of grazing incidence x-ray scattering in conjunction with atomic force microscopy. Surface roughness parameters and correlation lengths of surface height fluctuations obtained by these two different methods are compared. Further, a statistical correlation between the average grain size and lateral correlation length has been found.

[DP01.91] Materials: Synthesis, Growth, and Processing by using Surface Science Techniques

Surface science techniques ae very important to the materials preparing and characterization. Many useful solid-state devices can be synthesized by using MBE, MOCVD methods. By using surface detection technique, we have obtained interesting information on the atomic size and the mechanism of Si surface reconstruction. We can propose new designing view point of the material that will be used in the future. Using XPS, we have analyzed the composition of Ga atom on Si surface. To imporve the electronic properties of the devices, various substrates such as semiconductor (Si, SiC, GaAs, GASb etc), Metal (Au, Pt, Ag, etc), and insulant have been used to extend the detection range of the sensor, the temperature tolerance range, and the heat dissipation ability.

[DP01.92] Tribology of Tungsten Trioxide Thin Films

We have made a tribocharacterization of WO_3 thin films. Crystalline, polycrystalline, and amorphous films grown on sapphire substrates using molecular beam epitaxy were studied. We employed both plowing and indentation tests. For the plowing tests we made nanoscratches in the surfaces using an atomic-force microscope (AFM) with high-spring-constant cantilevers with three- sided pyramid diamond tips. Indentations were made with a commercial indenter with a diamond Berkovich tip. Creep and pop-in were significant complications for quantitative analysis of indents, especially in the polycrystalline and amorphous films. Scans of the indents show a correlation between pop-in during the indentation and pile-up around the resulting surface modification.

[DP01.93] The non-perturbative spectral theory of surface-corrugated electron waveguides

The electron wave propagation through a 2D strip with randomly rough boundaries has been treated within the exact scattering operator approach which is non-perturbative in either roughness heights \zeta or slopes. We reveal a universal integral resonant rule (UIRR) that governs the behaviour of the disorder-induced dephasing and attenuation of waves. It says that the surface scattering `redistributes' the primary wave mainly into `resonant' normal modes, for which the transverse wavelengths are \sim\zeta, while the total number of the modes is \propto\zeta^-1. This fact has striking consequences, e.g. the dephasing in samples with low perturbations, (k\zeta)^2 \ll 1, dominates over the attenuation and exhibits a non-analytic (square-root) dependence on \zeta^2. UIRR gives rise to another surprising effects: re-entrant transparency and `phase' acceleration of the primary signal.

[DP01.94] Multilayer Cu-based Films for the Gyroscope Housings of the gravity Probe B Relativity Mission

Multilayer Cu-based films have been manufactured using magnetron sputtering on fused quartz substrates including the gyroscope housing electrodes. A 100 nm Ti layer was used to provide good adhesion to the fused quartz. A 200 nm Ti layer was used to overcoat the Cu film, to reduce electron emission in high electric fields and to protect the easily corroded copper film. The seven layer Cu-based films, with smooth surface, low stress, good adhesion and low resistivity have been successfully applied to the electrodes of the gyroscope housings. The gyroscopes have been spun to 170 Hz for more than 100,000 hours testing.

[DP01.95] Luminescent and Structural Properties of Terbium Doped ZrO2 Films Prepared by Pneumatic Spray Pyrolysis Technique.

Terbium doped ZrO2 photoluminescent films have been prepared by pneumatic spray pyrolysis deposition process. The substrate temperature and the doping concentration in the start solution were varied. It is observed that crystallinity of the films depends upon the substrate temperature. For a substrate temperature lower than 400 C, the deposited films are highly disordered, while for a temperature higher than 400 C, the crystalline structure of the prepared material presents the tetragonal phase. The excitation and emission spectra have been obtained. It is observed that a concentration quenching of the photoluminescent at values of activator concentration is above the 10 atomic percent(a/o), in the spraying solution.

[DP01.96] Cluster on surface: stability and diffusion

The structure, stability, and mobility of a highly symmetrical cluster on a surface were studied by using molecular dynamics calculations with a simple Lennard-Jones potential approach for modeling the cluster, surface, and their interactions. We used this approach to predict the thermal properties of the supported cluster on the surface in a wide range of temperature. In addition to the relationship between the stability of the cluster and the surface properties, we also examined the criteria for the cluster to be stable on the surface, factors including lattice mismatch, temperature, and relative interaction strength. In the cluster mobility study, we found that the cluster can diffuse rather rapidly on the surface. Particularly, cluster diffusion by rolling on the surface was observed. In addition, the diffusion constants were found to be strongly affected by the mismatch of atomic size between the cluster atom and surface atom.

[DP01.97] First Principles Studies of Facets on Mo(111) and W(111) Induced by Ultrathin Films of Rh

It is observed (T.E. Madey et al., Surf. Rev. Lett. 3), 1315(1996).that atomically rough and stable Mo(111) and W(111) surfaces undergo massive reconstruction from a planar morphology to a microscopically faceted surfaces upon annealing when covered by films \ge 1 physical monolayer(PML) thick of Rh. The faceted morphologies are found to comprise 3-side pyramids having \211\ facets. Using first principles calculations, we calculated the surface energies of Mo and W in the (111) and (211) orientations before adsorption and after covered by 1PML of Rh film. The results show that the surface energies of clean surfaces in the (111) orientation are lower than that in the (211) orientation, thus the (111) surfaces are stable against the (211) surfaces. However, after adsorbing 1PML of Rh, the situations are changed. For both systems of Rh/Mo(111) and Rh/W(111) , the \211\ facets induced by Rh can coexit with (111) planar surfaces, and the onset annealing temperature for Rh to induce faceting should be higher than that for some other metal overlayers such as Pd.

[DP01.98] Interconnected Ballistic Quantum Wires

We present a model calculation of the electronic transport properties of a 2D quantum ballistic system formed by two parallel and interconnected quantum wires. The connection is modeled by a delta-function barrier along the common wall of the two quantum wires, that allows the electrons to migrate from one quantum wire to the other. The system as a whole is connected to reservoirs through a four terminal set. We solve an effective mass Schrödinger equation, using a mode matching tcchnique for independent electrons, to find the wave function, the current density, and the reflection and transmission coefficients in all four terminals. This allows us to obtain all the different non-linear resistances that can be defined for multiterminal systems such as ours, and that are also necessary to compare our results with experimental observations available in the literature.

[DP01.99] Effects of Magnetic Fields in Two-Dimensional Quantum Rings

We consider independent electrons confined to the interior of a planar ring under the influence of external magnetic fields perpendicular to the ring itself. We calculate the eigenenergies, and the eigenfuntions of this system based on an effective mass hamiltonian, in order to find the magneto-optical transitions and their oscillator strengths as a function of the strength of the magnetic field applied, and the interior and exterior radii of the confining ring.

[DP01.100] Many-Body Wave Function for a Quantum Dot in a Weak Magnetic Field

The ground states of parabolically confined electrons in a quantum dot are studied by both direct numerical diagonalization and quantum Monte Carlo (QMC) methods. We present a simple but accurate variational many-body wave function for the dot in the limit of a weak magnetic field. The wave function has the center-of-mass motion restricted to the lowest energy state and the electron-electron interaction is taken into account by a Jastrow two-body correlation factor. The optimized wave function has an accuracy very close to the state-of-the-art numerical diagonalization calculations. The results and the computational efficiency indicate that the presented wave function combined with the QMC method suits ideally for studies of large quantum dots.

[DP01.101] Collapse of Fano resonances in a quantum nanowire

We investigate the resonance structure of the electron transmission through a quasi-one-dimensional constriction with an attractive, finite-size impurity. In this system, two distinctive types of resonance structures, the Breit-Wigner resonances and the Fano line shapes, are observed in the same energy window. We show that the Fano resonance and antiresonance collapse at a critical energy and critical impurity size due to the appearance of localized electron states in the continuum. Finally, we predict a novel coherent effect, a swing or switching of the Fano resonance features in the transmission.

[DP01.102] Effective potential calculation of electronic states in v-grooved GaAs/AlGaAs quantum wires

In this work we propose a simplified effective potential scheme which readily provides electron and hole energies and wave functions for v-grooved or ridge quantum wires. The method consists in a coordinate transform followed by a tentative effective potential which produces a set of two decoupled 1D Schrödinger equations. The potential profile is transformed from its v-shaped form into the geometry of a simple quantum well [T. Inoshita and H. Sakaki, J. Appl.\ Phys. 79, 269 (1996)]. The effective potential is of the type V\tanh^2(x/b), where b is the length of the lateral potential well, and replaces all crossed terms of the tranformed 2D Schrödinger equation. In this way we obtain two decoupled Schrödinger equations which can be solved straightfowardly. Our results show an excellent agreement for energy levels and wave functions when compared to more complicated theoretical calculations.

[DP01.103] Phonons in Si dots

Phonons in Si dots were investigated by a lattice-dynamics calculation. Phonon modes in dots were studied in the real and reciprical space and compared with crystal modes. Their size depedences were discussed. The specfic heat of dots was found to differ significantly from the crystal value due to the finite size effect.

[DP01.104] Semiclassical dimensional perturbation theory for quantum dots

Dimensional perturbation theory is applied to the two electron quantum dot, obtaining highly accurate values for ground and excited state energies. The expansion parameter is 1/\kappa where \kappa = D+2|m|-1, D is the spatial dimensionality and m is the azimuthal quantum number. In this method, no approximations are made in its treatment of correlation. Analytic approximations for ground and excited state energies are obtained from the zero and first order terms of the perturbation expansion. For quantum dots with a small confinement region, this approximation is in good agreement with ``exact'' numerical calculations with relative error better than 3%. These first two analytic terms of the perturbation expansion constitute a semiclassical approach to the quantum dot from a perturbation formalism. Systematic corrections are made to the semiclassical approximation by adding higher-order perturbation terms where Padé approximants are used to calculate the partial sums of the energy series. The method can be extended to obtain an analytic approximation to the ground state energy of the many-electron quantum dot Hamiltonian for an arbitrary number of electrons, N, by truncating the 1/D expansion to low order.

[DP01.105] Dielectric Response Properties of an N Quantum Wire System Embedded in a Semi-infinite Host Medium

We have examined the nonlocal, dynamic dielectric response properties of an N quantum wire system embedded in a semi-infinite plasma-like host medium. This has been done within the framework of the random phase approximation, and the geometric dependencies on z_0, the distance of the first quantum wire from the bounding surface, and on the seperation of the quantum wires has been studied. In particular, the coupled mode dispersion relation for the plasmons of the N quantum wire system in interaction with the surface and bulk plasmons of the host material are analyzed for dependencies on the geometrical parameters.

[DP01.106] Mesoscopic Rings and Boundary Conformal Field Theory

We present a method to treat the effects of impurities and electron-electron interactions in mesoscopic normal rings using boundary conformal field theory. We derive the general form of the partition function and use it to calculate the finite temperature persistent current developed in the ring as a function of the magnetic flux threading the ring. These results are shown to agree with experiments.

[DP01.107] Coupled Mode Dispersion Relation for Double Quantum Wire Plasmons with Surface and Bulk Plasmons of Host Medium

We have determined the inverse dielectric function K(\vecx_1,\vecx_2;ømega) of a double quantum wire (DQW) system embedded in a semi-infinite plasma-like host medium at a distance z_0 from its bounding surface. This has been carried out using the random phase approximation (RPA) in position representation for thin quantum wires neglecting tunneling (but including the role of several subbands) and the semi-infinite host plasma is considered in the local limit. We have obtained an explicit closed form result for K(\vecx_1,\vecx_2;ømega) including its depedencies on z_0 (distance of DQW system from interface) and on the seperation of the quantum wires. In particular, we have analyzed the coupled mode dispersion relation for DQW plasmons in interaction with the surface and bulk plasmons of the host medium as it depends upon the geometrical parameters, z_0 and the quantum wire seperation.

[DP01.108] Excitonic Spectrum of a Quantum Dot Superlattice

We investigate optical properties of a quantum dot superlattice in the presence of electric and magnetic fields aligned with the superlattice axis. We have calculated the optical absorption in this system by solving the electron-hole effective Hamiltonian. For zero magnetic field, we have found ( M. Pacheco and Z. Barticevic, Phys. Rev. B 55), 10688 (1997). that the excitonic spectrum is governed by the competing action between the different quantum confinement mechanisms: the geometric and the Stark confinements. The presence of a magnetic field introduces an aditional confinement which modifies the lateral characteristic energies. The excitonic spectrum presents a rich structure of resonances related with "intradot" and "interdot" excitons. The ratio between the characteristic energies associated with the electric field and the effective lateral confinements determine the main features of the absorption spectrum.

[DP01.109] Theoretical aspects for designing a quantum well mirror

We have studied the real and imaginary parts of the dielectric constant of an Al_xGa_1-xAs/AlAs multiple quantum well structure as a function of the aluminum concentration and an externally applied electric field. Physical requirements for designing and operating quantum well mirrors are obtained. To make use of the direct transition properties of the Al_xGa_1-xAs/AlAs materials, the width of the quantum wells should be less than 100 Angstroms with x < 0.41. For operating quantum well mirrors, the magnitude of the applied electric field should be strong enough to allow the radiative transitions to ocurr, but not too strong so that absorption of the incident radiation is minimum.

[DP01.110] Single-electron devices and nanolithography using \mboxcalixarene

High-resolution low-energy electron-beam lithography using the negative electron resist calixarene allows the definition of device dimensions down to 10 nm at an electron energy of only 2 keV (A. Tilke, M. Vogel, F. Simmel, A. Kriele, R. H. Blick, H. Lorenz, D. A. Wharam, and J. P. Kotthaus, submitted to Journal of Vacuum Science and Technology B). Since the electron beam exposes the active area of the defined device, possible irradiation damages during the electron-beam lithography need to be considered. We show, that these damages are not significant in low-energy electron-beam lithography using calixarene. Moreover, we report on the preparation of single electron devices in silicon-on-insulator films in the sub 10 nm range. We achieve a writing resolution of 10 nm. After the electron-beam lithography, the silicon-on-insulator film is etched with a reactive ion etching step using CF_4. Device dimensions below 10 nm are demonstrated by shrinking the effective dimensions of the structures in a thermal oxidation process. Due to the absence of proximity effects in low-energy electron-beam lithography we succeed in defining multiple dot structures in the 10 nm regime. We show first results on low temperature Coulomb oscillation measurements on these devices.

[DP01.111] Pb Nanowires in Confined Geometry

The study of materials with dimensions at nanometer scales or in confined structures have recently attracted great interest since many bulk properties of the matter can change under those conditions, these facts open the possibility of studying new interesting phenomena with possible applications of these novel materials. In the present work we report on the synthesis of nanowires of lead with a mean diameter of 30nm confined into nanoporous af aluminum oxide templates. We studied the Pb nanowires by means of high resolution electron microscopy. Our main result consists on the finding that the crystalline structure of Pb is not the usual fcc bulk structure, but one that can be associated to a tetragonal cell with lattice parameters a=4.1A and c=4.5A. Magnetization versus temperature measurements of the Pb nanowires confined into the pores shows a superconducting transition temperature around 7.1K, a somehow lower critical temperature compared to bulk Pb. We will discuss our experimental results on the view of the structural restrictions on Pb produced by the confinement.

[DP01.112] Dipole-Dipole Correlation Function for an Electron Plasma Bound in a Harmonic Potential

We have constructed the RPA integral equation for the inverse dielectric function for an electron plasma bound in a harmonic potential in the presence of a quantizing magnetic field. Particular attention is given to the dipole-dipole correlation function, for which we obtain a closed-form solution in integral representation. All frequency poles of the dipole-dipole correlation function are collective modes of the confined magnetoplasma(but do not necessarily encompass the whole spectrum). These modes are analysed numerically using the integral representation.

[DP01.113] Fast Spectroscopic Measurements of the Unwinding Kinetics of DNA Hairpin Molecules

The mechanism of sequence-specific recognition of DNA binding sites by proteins involves conformational changes in both proteins and DNA upon binding. There is some evidence that suggests that DNA sequences that render the duplex more unstable result in an increased affinity for proteins. One explanation of this result is that proteins bind preferentially to a transient state along the helix-coil transition pathway which most resembles the state of the DNA bound by the protein or which is perhaps locally unwound and more susceptible to a conformational change. In order to explore this hypothesis of DNA-protein interactions, it is necessary to examine in detail the helix-coil transition pathway and to identify and characterize possible intermediates. The thermodynamics of helix-coil transitions has been extensively studied. However, the dynamics are much less well characterized. We have undertaken a systematic investigation of the unwinding kinetics of short DNA chains. The unwinding is initiated by a 10-ns laser-induced temperature-jump and the kinetics are monitored using time- resolved absorption spectroscopy. Results will be presented on the kinetics of unwinding of hairpin loops formed from sequence isomers of 5^\prime-GGATAA(TTTT)TTATCC-3^\prime where the central TTTT forms the loop.

[DP01.114] The Physical Mechanism in Receptor Dimerization, Clustering, and Adaptation

Intracellular signaling often arises from ligand-induced dimerization or oligomerization of cell surface receptors. This oligomerization or clustering process is fundamentally a cooperative behavior between near-neighbor receptor molecules; the properties of this cooperative process clearly affects the signal transduction. Moreover, the adaptation of intracellular signaling can affect the dynamics of receptor clustering as well. Recent investigations have revealed the molecular basis of receptor-receptor interactions, but a simple theoretical framework for using this data to predicting cluster formation and the polymorphic cell adaptation process has been lacking. Here, we propose a simple, coarse-grained, phenomenological model for ligand-modulated receptor interactions and discuss its properties via mean-field theory. The model has only two effective parameters; one is a fast field describing receptor clustering and another is a slow one for adaptation process. Results of the model unify a number of experimental findings. Moreover, the existence of a first-order transition for this model has immediate implications regarding the excitable robustness of the cellular signaling response.

[DP01.115] Spectral Characteristics and Structure of Short, Single-stranded, 2-aminopurine-containing Oligonucleotides

Purified di- and trinucleotides, 5'PT, APT, GPC, and CPG, containing the fluorescent base, P= 2-aminopurine, have been prepared and studied using optical spectroscopy and NMR. Changes in stacking interactions in the single strands are measured by absorption and 2D fluorescence spectroscopy, at wavelengths characteristic both of 2-aminopurine and of the normal bases. Careful subtraction of reference spectra from temperature-dependent spectra allows quantitation of stacking changes with temperature. Time- and temperature-resolved fluorescence decay components characterize distinct, but structurally ill-defined conformations. NOESY spectroscopy then determines the presence of one or two specific conformations and allows correlation of optical spectral and decay components with structure. Comparisons with previous results on less highly-purified oligonucleotides will be made.

[DP01.116] A New Nonlinear DNA Model.

The torsional dynamics of DNA can be described by nonlinear models(Yomosa, S., Phys. Rev. A, \b27), 2120 (1983).(Yakushevich, L.V., Phys. Lett. A, \b136), 413 (1989).(Gonzalez, J.A. and Martin-Landrove, M., Phys. Lett. A, \b191), 409 (1994). , predicting solitonic open states related to replication and transcription processes. In particular, the Yakushevich model yields solitonic solutions with appropriate topological properties to describe those processes, as shown in reference 3. In the present work, we developed a model that combines both Yakushevich and Yomosa models, treating the stacking interaction between adjacent bases as a perturbation but keeping it as a nonlinear term. In this way, both transversal and longitudinal interactions are treated on the same foot. Stable solitonic solutions, energies and its dynamics were obtained.

[DP01.117] Lipase Catalysis at the Air/Water Interface

Lipase is an essential enzyme for in vivo digestion and storage of fats. One of the most characteristic and intriguing features of this lipolytic enzyme is the activation at interfaces. In this study, lipase hydrolysis kinetics, from Pseudomonas cepacia, in biomimetic monolayers at the air/water interface is examined. A surface active substrate, umbelliferone-stearate, is injected beneath the monolayer and its adsorption to the monolayer is fast compared to the hydrolysis rate and the products desorption from the monolayer is also fast. One of the products fluoresces in the subphase, hence the rate of hydrolysis of the substrate can be tracked by circulating the subphase buffer through a fluorometer. Lipase catalysis proceeds via the interfacial Michaelis-Menten mechanism.

[DP01.118] Complexity in DNA Genetic Code.

In the present work, we propose a new method to characterize the complexity of genetic code in DNA strands by using a specific measure of the entropy. The target DNA sequences were the same as those reported by Peng et al.(C.-K. Peng, S.V. Buldyrev, A.L. Goldberger, S. Havlin, F. Sciortino, M. Simons and H.E. Stanley, Nature, \b356), 168 (1992)., analyzing not only the original sequence but also its coding segments and proteins. Also intron-containing sequences were modified to simulate intron-less sequences for comparison. The results reveal that there is no a significant difference for the entropy measure between intron-containing and intron-less sequences, which seems to be in correspondence with other results(V.M. Prabhu and J.M. Claverie, Nature, \b359), 782 (1992), R.F Voss, Fractals, \b2, 1 (1994).. In the case of proteins, they exhibit a well defined behavior as a function of the length of the peptide chain. Further work is pursued to analyze a larger database even for nucleotide and aminoacid sequences.

\specialsrc: 278 COMPLEX.TEX

\specialsrc: 295 COMPLEX.TEX

[DP01.119] First-Principle Investigation of Hyperfine and Magnetic Properties of Horseheart Cytochrome c

We have used the first-principle Hartree-Fock-Roothaan Procedure, which has been found to provide good quantitative agreement for hyperfine constants in hemin (N.Sahoo et al, Theo. Chem. Acta \underline82), 285(1992), to study the charge and spin distributions in Horseheart Cytochrome c and the hyperfine properties of ^57F_e, ^33S, ^14N and ^1H nuclei. Our results will be compared with experiment (C.P. Scholes et al, Biochim. Biophys. Acta \underline 434), 290(1976) and J. Am. Chem. Soc. \underline108, 166(1986) and the earlier estimated values (J.N.Roy et al, J. Am.Chem. Soc. \underline105), 7729(1983) by the semiempirical Self-Consistant Charge Extended Hückel(SCCEH) procedure. The possible trapping locations for muon and muonium in the heme unit will be studied for interpretation of data from Muon Spin Rotation Experiments.

[DP01.120] Low frequency Raman scattering of IgG2a antibody

The Raman spectrum of the IgG2a antibody molecule is reported. In accordance with previous studies on immunoglobulins the secondary structure is predominantly of the \beta-sheet type. The low frequency region (< 100 cm^-1) of the Raman spectrum is analyzed in detail. A structured band centered at 80 cm^-1 is observed. This band is attributed to relative displacements of the subunits of the molecule or "accordeon-like" breathing motions. The results are interpreted using a more complex model than the usual vibrating sphere. This model considers the vibration of two long and two short rods representing the two heavy and the two light chains of the antibody, respectively.

[DP01.121] T-jump induced protein secondary structure change in bacteriorhodopsin

Laser-induced temperature jump (T-jump) technique and nanosecond time-resolved FTIR spectroscopy have been used to study the secondary structural changes of bacteriorhodopsin (bR). The helical structural changes and the hydrogen/deuterium (H/D) exchange are triggered by using a 15C T-jump from a preheated bR D2O solution at 72C. The structural transition from aII-helical to alpha I-helical is observed, by following the change in the frequency of the amide I band (C=O stretching mode) from 1667 to 1651 cm-1. Two rise times (\sim 65 ns and \sim 394 ns) for this conversion are found, indicating either two sequential steps in the pre-melting process or two independent pre-melting processes for two different parts of bR. These results suggest that the T-jump induced alpha II to I structural interconversion is fully reversible between laser pulses, and that this fast interconversion falls in the "pretransition" temperature domain on the melting curve of BR.

[DP01.122] Computer-Aided Genetic Engineering of Bacteriorhodopsin

Bacteriorhodopsin (BR) is an active transport protein found in the plasma membrane of the salt-loving bacteria Halobacteria salinarium, which is being considered as a holographic data storage medium(R.Birge,Scientific American) 272, 90 (1995).. The chromophore retinal is bound to Lys216 of BR through a protonated Schiff base linkage. On absorbing visible light, retinal will isomerize from its all-trans state to its 13, 11, or 9-cis isomer. The 13-cis form is involved in the BR photocycle while the 11-cis form is involved in the vision pathway. After isomeration to the 13 or 11-cis form the molecule is able to return rather quickly to the all-trans state. In contrast, after isomerization to the 9-cis form, the molecule remains in that state, making the 9-cis state preferable for permanent data storage.

Quantum chemistry and molecular dynamics calculations are being used to identify specific mutations within the BR molecule that hamper rotations to the 11 and 13-cis states while favoring isomerization to the 9-cis state. In the first part of the project, quantum chemical calculations are being used to examine ground and excited state pathways for all three double bond rotations. We want to determine if additional intermediate states are involved, what the energy barriers for rotation are, and how the electron density changes in going from trans to cis in each system. Also, the effect of the Schiff base linkage on rotation around each double bond is being examined.

[DP01.123] First-principles Study on Electronic Structures of \beta-Pleated Sheets in Proteins

The electronic structures of the typical secondary forms of proteins, \beta-pleated sheets, are theoretically investigated by first principles calculations. By staggering the individual peptide chains in parallel (\beta_P) or in antiparallel (\beta_A), these secondary forms stabilize their energies from those of single chains by 32.2 and 13.5 kcal/mol (per glycine residue), respectively. The interchain bridging H bonds play an important role in this condensation. At the band-edge states, however, H hybridization does not occur, and electrons or holes can not be delocalized toward the interchain direction. The band-edge states are, rather, controlled by the folding of the peptide skeleton. This folding interchanges the \pi and \sigma characters at the valence band edge, and generates the delocalized sp\sigma character at the conduction band edge. These features are also found in cysteine \beta-pleated sheets. We will report the electronic role of disulfide bonds connecting two peptide chains.

[DP01.124] Protein Synthesis and Secondary Structures Formation.

Protein synthesis precedes the formation of molecular structures. Secondary structure is determined by short range interactions between the component aminoacids and solvent molecules. Most of the published papers do not include these facts. In the present work, it is proposed a method that considers the interaction of the protein with the solvent and in which the structure depends on the synthesis. The potentials considered for the calculations are of the semiempirical type, with the simplification that each aminoacid is considered a rigid sphere. This assumption take us to consider the multipolar moments, in a way that less information of the internal aminoacid structure is lost. Only dipolar moment was considered. The energy minimization at each step was performed by Montecarlo methods to explore the configurational space nearby the optimized structure obtained in the preceding step, prior to the addition of another aminoacid. The results of these simulations were validated using molecular dynamics, obtaining a good agreement regardless of all the simplifications made in the model. The results show a strong dependence of the folding process with synthesis and solvent interaction.

[DP01.125] Probing Retinal Excited-State Dynamics in Bacteriorhodopsin on the Femtosecond Timescale

The decay of excited state retinal in native bacteriorhodopsin (bR) in 500 fs suggests a barrierless process (photoisomerization) that is catalyzed by the protein. Using femtosecond transient spectroscopy, we carried out experiments to answer the following questions: 1.) Is the decay of the retinal excited state in bR indeed barrierless? 2.) How does the protein catalysis of the excited state decay of all-trans compare with that of 13-cis retinal? The fact that the decay lifetimes of bR at room temperature and at 10 K are comparable strongly suggests that the 0.5 ps decay is barrierless. Comparison of the decay lifetimes for the all-trans and 13-cis isomers in native bR with those in either the D85N mutant or deionized bR suggests that the protein environment of bR catalyzes the rate of decay of the all-trans isomer by 300% and the 13-cis isomer by >2000%.

[DP01.126] Mechanism of Formation of Supported Phospholipid Bilayers by Fusion of Unilamellar Vesicles Investigated by AFM.

Fusion of small unilamellar vesicles (SUVs) was originally used to prepare protein-incorporating supported phospholipid bilayers (SPBs) for studies of cell-cell interaction processes. Due to their unique properties, SPBs prepared in this fashion have subsequently found applications in other fundamental (from structural biology to physics) and applied (surface modification, biosensor technology) research fields. No consistent picture of the mechanism by which SPBs are formed has emerged, however. Arguments concerning stressed nature of SUVs (which are usually used to prepare SPBs) are resorted to whenever the driving forces for SPB formation are discussed, in spite of abundant evidence that SPBs can be formed equally well from vesicles obtained by extrusion or dialysis. Several new features of the process of SPB formation by fusion of unilamellar vesicles revealed by AFM will be presented and interpreted within the theoretical framework due to Seifert et al. (1),(2).

1. Seifert, U. Adv. Phys. (1997), 46, 13. 2. Reviakine, I., Brisson, A., submitted.

[DP01.127] Effect of monovalent ions on phase behavior of multilamellar vesicles

The effect of small amounts of alkali halide salts on the behaviour of multilamellar phospholipid vesicles close to the main transition has been studied using three different experimental techniques - Small-Angle X-ray Scattering (SAXS), Differential Scanning Calorimetry (DSC) and Piezoelectric Bulk Modulus Gauge (PBG). The vesicles are composed of one phospholipid species from the series DC_nPC. The samples are prepared in excess water condition and the salt concentration ranges from 0 to 200 mM.

Using SAXS, we observe that in the fluid phase well above T_m, the repeat distance is shifted. The anomalous swelling in repeat distance for T \rightarrow T_m^+ is changed dramatically. By DSC, it is primarily observed that salts shift the main transition temperature. Using the PBG, the velocity of sound in a sample is measured with great precision by producing a standing wave in a spherical geometry.

So far, most effects observed may be explained in terms of the anomalous swelling as the proceeding of a critical unbinding of the lamellae upon approaching the main transition temperature. We suggest that addition of salt alters the proximity of the critical temperature of the unbinding to the main transition temperature by changing the Hamaker constant and the decay length of the hydration force.

[DP01.128] Measurement of Adsorption and Desorption of Colloidal Particles at Water-Glass Interface

Using a technique combining evanescent field fluorescence with video-microscopy we study the sorption of colloidal particles from flow in a parallel plate channel on a glas surface. In the present contribution we discuss the possibilities and limitations of this technique, focussing thereby on adsorption and desorption experiments of negatively charged poylstyrene particles onto an equally charged glass surface. The glass surface, though optically flat and well cleaned, is not homogeneous, but rather the sorption occurs at a limited number of preferred sites.(Lüthi, Y., Ri\vc)ka, J., Borkovec, M., J. Colloid Interface Sci. 206, 314, 1998

[DP01.129] Critical Exponent \alpha and Thermodynamic Non-equilibrium in Lipid Bilayers Containing Sterols

The (pre-) critical exponent \alpha has been measured in lipid bilayers containing cholesterol and androsten, from small-angle x-ray diffraction measurements on the variation of bilayer thickness with temperature T (Richter et al. Physical Review E, submitted) and over an effective scan rate dT/dt varying by a factor of nearly 10,000 down to 0.1 mK/min. The increase in \alpha with increasing scan rate is compared with the effect due to an increasing sterol concentration. The data suggest that the exponent could be viewed as a measure of non-equilibrium. These measurements underscore the experimental importance of slow experiments, even though they may be expensive in machine time. Supported by NATO CRG 970225.

[DP01.130] Single Molecule Detection of Hemoglobin by Surface-Enhanced Raman Scattering (SERS)

We report single molecule detection (SMD) of hemoglobin (Hb) by SERS. A silver hydrosol was incubated together with a dilute solution of human adult met-Hb in order to obtain an average of 0.25 Hb molecules per Ag particle. Hb/Ag aggregates were immobilized on polymer-coated glass or Si surfaces and investigated using confocal Raman spectroscopy (5145 Åas well as Scanning Electron and Atomic Force Microscopy (SEM amp; AFM). Spectra from single Hb molecules could be obtained from only a few of the investigated sites. Investigations by SEM and AFM revealed that these "hot sites" were composed of pairs of closely spaced Ag-particles with typical dimensions of \sim100 nm. Moreover, the intensity of the Raman spectra reached its maximum when the polarization of incident light was parallel to the pair-axis. These results can be understood from classical electromagnetic theory, if one assumes that the Hb molecule is situated between the two Ag particles. Extinction spectra of Hb/Ag sols, which showed that Hb caused rapid aggregation, indicate that this is the case.

[DP01.131] A Study of the Production and Spatial Distribution of Radicals in Cleaved and X-Irradiated Hard Keratin

Perthiyl and carbon centered radicals, are produced in the hard keratin of horn, feather, or hair by cleaving or X-irradiation. The formation of stabilized radicals in cleaved keratin appears to be uniquely related to the keratin structure. EPR does not detect stabilized radicals of cleaved amino acids, the components of keratin, or of cleaved polymers, such as wood or teflon. We assume that cleaving of keratin breaks bonds, leaving unpaired electrons that stabilize almost equally on sulfur containing residues and carbon atoms in the protein chain to which the glycine residue is attached. Diffusion of water into the samples was studied in order to obtain data on the spatial distribution of the radicals. When cleaved samples are allowed to age in a water bath, approximately 80% of the radicals are lost in the first 15 seconds, while for X-irradiated samples the loss is 30%. These observations indicate that radicals in cleaved samples are located near the surface while those in X-irradiated samples are distributed more uniformly. An analysis of the diffusion data indicates that 80% of the radicals from cleaving reside within 0.2 microns of the surface. Optical spectra of irradiated samples will be discussed.

[DP01.132] Aggretation Events in Interleukin-1beta Depend Upon the Characteristics of a Kinetic Folding Intermediate

A point mutation, lysine 97 to isoleucine (K97I), in the beta-sheet protein interleukin-1beta (IL-1beta), results in a dramatic increase in inclusion body formation. The folding pathway of IL-1beta was studied with multiple kinetic and spectroscopic methods to determine whether the properties of folding intermediate(s) in vitro correlate with aggregation in vivo. Light scattering experiments indicate that aggregation, absent in wild-type IL-1beta, will occur in the K97I variant above 18 micromolar, consistent with a nucleating step prior to aggregation. The kinetics of aggregation fit well to a mechanism where a transiently populated intermediate is involved in the association phenomenon. This intermediate shows amide exhange protection in the central beta-strands but leaves a number of hydrophobic regions exposed. The hydrophobic K97I mutation directly increases two of these regions prior to their burial in the native state, thereby increasing the likelihood of intermolecular association from this intermediate.

[DP01.133] Exploring the Folding Funnels of an Off-Lattice Minimalist Model

We investigate the folding thermodynamics and explore the Hamiltonian of funnel-like folding in an off-lattice minimalist model. We have performed molecular dynamics simulations on a four helix bundle native structure as a framework that can quantitatively investigate the relationship of cooperative folding between secondary and tertiary structures. We explore what physically relevant order parameters are more appropriate to utilize thermodynamic data to explain the folding kinetics. The dependence of the free energy surfaces as a function of these different progress coordinates will be carefully discussed.

[DP01.134] Structure and Dynamics of Surfactant Stabilized Aggregates of Palladium Nanoparticles under Dilute and Semidilute Conditions - A Study Using Static and Dynamic X-ray Scattering

We have used high resolution small angle x-ray scattering and the recently introduced technique of x-ray photon correlation spectroscopy to study the structure and diffusive dynamics of a colloidal palladium aggregate sol under dilute and semidilute conditions. At low concentration we find that the size of the aggregates as determined from the static structure factor and from the diffusive dynamics agree. Structural changes induced by increasing the concentration can be attributed to inter-aggregate correlations corresponding to partially overlapping aggregates. At high concentrations the apparent diffusion constant decreases, while the system remains still in a liquid-like state. The comparison of static and dynamic data gives insight into structural features which are indiscernable by one technique alone.

[DP01.135] A Transient Electrically Induced Birefringence Study of Synthetic Clay Platelet Aggregation

Aggregates in water of the synthetic monodisperse model clay system Laponite RD has been studied by means of Transient Electrically Induced Birefringence Measurements. We have investigated the isotropic liquid phase (IL)(A. Mourchid et.al. Langmuir 14, 4718 (1998)) at various electrolyte (NaCl) and clay concentrations. We find multiple relaxation times at low electrolyte concentrations and near single and longer relaxation times at higher NaCl concentrations as the flocculated (F) phase is approached. We may thus at this point suggest that the aggregate sizes increase with increasing electrolyte concentration as expected, and also that the aggregate size distribution narrows as the average size increases. Further experimental investigations are necessary and ongoing in order to answer questions related to whether the electric pulse simply rotates roundish or maybe near linear aggregates(M. Dijkstra et.al. Phys.Rev.E55, 3044 (1997))(F. Pignon et.al. Phys.Rev.E56, 3281 (1997)), deforms these aggregates or maybe destroys aggregates thus possibly inducing transient reaggregation.

[DP01.136] Isotopic Interdiffusion in the Grand Canonical Ensemble: Theory, Subsystem Analysis, and Simulation

We study the interdiffusion of two chemically identical speciesof different masses in the grand canonical ensemble in order to eliminate the artificial conservation of total momentum. Specifically, we consider a cubic system of volume L^3 in the microcanonical ensemble and analyze the dynamics within a subsystem of volume \ell^3; particles, energy, and momentum are exchanged at the subsystem boundary. The time derivative of the structure-function matrix can be expressed in two forms: a phenomenological expression involving the diffusion matrix and an expression derived from a Mori-Zwanzig projection. Comparison of the two expressions yields an equation for the diffusion matrix in terms of the static structure function matrix and a matrix of Onsager coefficients. We compare this equation with simulation results and discuss finite-size effects.

[DP01.137] Stability of Dielectric Fluids in an External Electric Field

We examine the stability of superposed fluids with different dielectric constants in an external electric field. We find that the electric field has a stabilizing effect on the configuration and can be used to augment surface tension in controlling surface reactions and crystal growth. We suggest several experimental configurations that could be used to test these initial theoretical results and discuss future nonlinear computations.

[DP01.138] Phase Separation of Binary Nematic Liquid Crystal Mixtures

We investigated the phase separation process of a 5CB-MBBA mixture of various concentrations. Initially the mixture is homogeneous at the isotropic phase. By lowering the temperature, the phase process occurs which is observed by means of a polarizing microscope. In addition, the phase transition temperature of each mixture was measured by using a cloud point method and a transition temperature versus concentration phase diagram was established. This allowed us to determine the critical composition. The formation of droplets in the two-phase region is being monitored as a function of concentration and temperature. This research was supported by NSF grant DMR 9619406 and The College of Wooster.

[DP01.139] Probing molecular orientations and conformations at the free surfaces of liquid crystals

Sum-frequency vibrational spectroscopy was used to probe molecular orientations and conformations of the free surfaces of cyanobiphenyl (nCB, with n ranging from 2 to 11) liquid crystalline media. In both isotropic and liquid crystalline phases, the alkyl chains were found to be projecting out from the liquid crystalline surface. Spectral analysis of the C--H stretch modes from different input/output polarization combinations indicated that the chromophores of the molecules are oriented more or less along the surface normal with a broad distribution. As the alkyl chain length increases, there are more trans-gauche defects appearing along the chains. The close similarity of the spectra for different nCB however suggests that the average surface molecular orientations of all nCB are very much the same.

[DP01.140] Investigation of Electric Field Induced Wall and Zigzag Structures in the Nematic Liquid Crystal 5CB

A homeotropically aligned nematic liquid crystal is subjected to the action of an ac electric field applied in the sample plane. With progressively increasing electric voltage, walls move away from the electrodes, approach each other and merge. The walls are decorated along their length by a zigzag defect pattern which was reported recently [1]. Here we report the temperature dependence of the wall position as a function of applied voltage. The zigzag structure found adjacent to the electric field induced walls was monitored as the sample was rotated between crossed polarizers. The average length of each element of the zigzag defect was plotted as a function of the angle of rotation. Observations are explained qualitatively. This research was supported by NSF grant DMR 9619406 and The College of Wooster. [1] Shila Garg, Erica Bramley and U. D. Kini, Molec.Cryst.Liq.Cryst (in press).

[DP01.141] Theory of Chiral Smectic-C Liquid Crystals (SmC*): Spontaneous Periodic Textures in SmC* Films

The free energy density of SmC* is constructed. The new equations of motion for the smectic order parameter are derived. They represent a chiral generalization of the equations which describe post-critical Rayleigh-Benard convection and spatially non-uniform resistive states in narrow superconducting channels. Specific of the chiral case is that the spatially non-uniform (periodic) states may spontaneously arise in equilibrium systems (freely suspended smectic films, Lengmuir monolayers, lipid membranes, etc.) as a result of chiral symmetry breaking. For the non-chiral cmectic-C phase the possibility of forming stripe modulations with alternating "left-hand" and "right-hand" C-phases separated from each other by kinks was predicted by the author \footnote JETP 76 , 245 (1993). In SmC* films analogous modulations are more complicated: strip texture being one-dimensional in the film plane (smectic planes) is essentially two-dimensional in the bulk of the film. We call such a stripe texture the lattice of chiral kinks. It is shown that arising the lattice has a threshold character with respect to increasing the film thickness h. The stripe periodicity D is much less than h, and D increases with increasing h. That kind of thereshold modulation has been experimentally observed in freely suspended SmC* films \footnote E.Gorecka et al., Phys. Rev. Lett. 75, 4047 (1995).

[DP01.142] Long Range Interactions and Nonextesivity in Power-law Fluids

Molecular dynamic simulations for systems with D=2,3 are studied. We assume that, at long distances, the two-body attractive potential decays as r^-\alpha. When \alpha > D (0\leq \alpha \leq D), thermodynamic extensivity (nonextensivity) is observed; Particular attention is payed to the liquid-gas critical point located, in the temperature-pressure plane, at (T_c,P_c). (T_c,P_c) are, in the N\rightarrow \infty limit (N\equiv number of molecules), finite for \alpha > D and diverge for \alpha \leq D. However, it is proposed a possible way for getting all quantities be finite for any value of \alpha. On the other hand, anomalous diffusion can be defined through the time dependence of the single particle mean square displacement \propto t^\gamma , where <.> is performed over different time origins and over all the particles, r the position and t the time. The transport is anomalous when \gamma \neq 1; in our case, superdiffusion (i.e., 1 < \gamma < 2) is observed when \alpha < 6. Finally, pair correlation functions are obtained directly from simulations. The trend of these functions shows the weight of the neighbor of every particle when long range interactions are considered. These results should be useful for discussing gravitation and some Casimir-like fluids.

[DP01.143] Turbulent transport, intermittency and statistical topography

The standard approach to turbulent transport of passive tracers (Lagrangian or Eulerian) focuses on the "ensemble" properties of the system, and exploits the tools of "statistical averaging" (means, moments, correlation, "turbulent diffusivity"). As such it often fail to capture the essential physics, relevant to "random states", or "realizations" of the process. Indeed, a typical "ensemble mean" represents a cumulative effect of "long temporal exposure" of tracer to stochastic input (random velocity), whereas the relevant time scale (or property of interest) could fall short of the "averaging scale". We propose an alternative view of such "stochastic dynamics" based on "statistical topography". The main focus is on evolution of "iso-levels" (contours or surfaces) of the advected density, and the corresponding PDFs. The levels, and related geometric parameters ("enclosed area" and/or "mass") provide the direct (physical space) description and measure of intermittency, as opposed to such indirect characteristics, as "normal" or "anomalous" turbulent exponents. In some cases (Gaussian delta- or short correlated fields), we get explicit Fokker-Planck equations for the "iso-level PDF", via the dynamic and stochastic analysis. These equations could be solved exactly or numerically, and produce a wealth of data. For instance, we show the formation and growth of intermittent structures (clustering) for compressible flows. Applications range from "floating tracers" to "wave-propagation and focusing" in random media.

[DP01.144] Coherent Spin Precession in Superfluid ^3He-B in the Nonhydrodynamic Regime: Experiment and Numerical Simulation

We present the systematic study of homogeneous spin precession in superfluid ^3He-B excited by cw NMR techniques below 0.4 T_c at a pressure of 6 bar. To stabilize the homogeneous precession at these conditions, a high NMR rf-field must be applied. Moreover, the shapes of the absorption and dispersion signals showed an unusual behavior when compared to the signals obtained above 0.4 T_c. This is probably caused by the contribution of the surface energy to the total energy of the system, leading to the so-called magnetic nonwetting regime(Yu. M. Bunkov et al.), Phys. Rev. Lett. \bf73, 1817 (1994). and a chaotic spin precession.(Yu. M. Bunkov et al.), Czech. J. Phys. 46 213 (1996). Preliminary numerical simulations, which take into account only a model for the magnetic nonwetting regime, show reasonable agreement with the experimental data.

[DP01.145] The motion of clumps and holes in a background vorticity distribution.

The time-scale for inviscid 2D turbulence to relax is often determined by the rate at which clumps drift up gradients in the background vorticity and holes drift down gradients. Here, we consider a clump or hole of circulation \Gamma_v in a background vorticity distribution that is initially axisymmetric and decreases monotonically with radius r. We find that clumps spiral toward the center of the distribution and that holes spiral outward. This can be understood from conservation of angular momentum L = \Gamma_tot \langle r^2 \rangle where \Gamma_tot is the total circulation and \langle r^2 \rangle is the total mean square radius of the flow. Both clumps and holes increase the mean square radius of the background through local mixing. Since clumps have positive vorticity, they must therefore decrease their radial position to conserve L; holes with negative vorticity must move outward. We calculate the velocity in the limit of small \Gamma_v, using a simplified model that linearizes the dynamics. We also use a vortex-in-cell simulation to compute the velocity at large values of \Gamma_v. These simulations show that the inward drift of clumps occurs more rapidly than the outward drift of holes as nonlinear effects become important.

[DP01.146] BEC in d dimensions for a free boson gas with a power-law dispersion-relation plus a boson gap

We report changes in the BEC T_c, its condensate fraction, and jump-discontinuity in specific heat caused by a possible (bosonic) temperature-dependent gap, \varepsilon_0(T), in the power-law dispersion-relation, i.e., \varepsilon(k) = \varepsilon_0(T) + c_s k^s, with s>0 and k the boson wave number, for an assembly of N free bosons. Without the boson gap \varepsilon_0(T), T_c is nonzero if and only if d/s>1, there is a specific-heat jump if and only if d/s>2, while the condensate fraction is 1-(T/T_c)^d/s. With a nonzero gap these properties are modified by the gap and its slope at Tc as follows: the minimum d/s for nonzero T_c decreases as \varepsilon_0(T_c) increases, there is a specific-heat jump for any d/s such that BEC exist, and the condensate fraction drops off less rapidly to zero at T_c.

[DP01.147] Proposed Measurement of Charged-Particle Heating on International Space Station

We present the analysis and design of a precision measurement of the bulk heating caused by the penetrating charged-particle fluxes of the low-Earth orbit environment. This measurement will be performed by direct calorimetry at low temperature, and will be incorporated into the DYNAMX experiment proposed for flight aboard the International Space Station. Results of the measurement will be useful to several upcoming high-accuracy, low-temperature orbital experiments. The charged-particle heating has been previously measured by Lipa and coworkers at lower altitudes and lower inclinations, but extrapolation of their data to the higher ISS orbit is not straightforward. This is due to both the complex interaction of the charged-particle fluxes with the Earth's magnetic field, and also to the complex effects of secondary particle showers caused by nuclear fragmentation. Our most recent estimates of the ISS charged-particle heating, which incorporate trapped-particle dose data from the recent APEX experiment near solar minimum, will also be presented. This work supported by NASA Microgravity Research Division.

[DP01.148] Localization, self-similarity and selection in conducting cooling flows

We investigate a late-stage conductive cooling of hot cavities (HCs) formed in a gas by a rapid local energy release. The cooling is enhanced by a self-consistent low-Mach-number inflow of the gas from the periphery under almost constant pressure. In the slab geometry, the cooling dynamics is describable by a nonlinear diffusion equation for the mass density \rho(x,t). In scaled units, \rho_t = (\rho^-1-\nu \rho_x)_x, where \nu is the exponent in the power-law temperature dependence of the heat conductivity. This equation describes strong localization of the HC in the process of cooling. There are many similarity solutions to this equation. Correspondingly, the size of the HC and temperature at its center can show a multitude of scaling behaviors. Therefore, an interesting selection problem arises. If the initial density profile shows a power-law behavior at large x, it is the exponent of this power law that selects a self-similar asymptotics(B. Meerson, Phys. Fluids A 1), 887 (1989).^,(D. Kaganovich, B. Meerson, A. Zigler, C. Cohen, and J. Levin, Phys. Plasmas 3), 631 (1996).. In this work we address the selection problem in the case of a more strongly localized initial condition: (a) exponentially or super-exponentially localized and (b) one with a compact support.

[DP01.149] New techniques for high resolution thermophysics experiments near the superfluid transition in ^4He

Due to its sharpness the superfluid transition in ^4He is one of the best physical systems to experimentally test theoretical models of critical phenomena. Since the natural temperature scale for these experiments is the deviation from the transition temperature T_\lambda, t=(T-T_\lambda)/T_\lambda, it is essential to have extremely good temperature stability and resolution near this point. We present lessons learned and new techniques devised in developing a low-temperature platform with sub-nano-Kelvin resolution near T_\lambda. Material discussed includes: - methods to improve temperature stability of 1K pot that is the prime source of the temperature noise;

- results from modeling of the multistage thermal network which includes multiple active temperature controls;

- new techniques of diffusion bonded, all-metal experimental cell construction.

[DP01.150] Characteristics of 2D Turbulent Flows that Self-Organize into Vortex Crystals.

The free relaxation of inviscid incompressible 2D turbulence is often dominated by strong vortices (intense vorticity patches) that move chaotically and merge. However, recent experiments have found that the mergers can be arrested by the formation of vortex crystals, in which a number N \sim 5-20 of strong vortices persist for long times, forming stable patterns in a low vorticity background.(K.S. Fine et al.), Phys. Rev. Lett. 75, 3277 (1995). Here we estimate N from properties of the initial flow. Vortex crystals form because the chaotic motions of the strong vortices ``cool'' as they increase the entropy of the background.(D.Z. Jin and D.H.E. Dubin, Phys. Rev. Lett. 80), 4434 (1998). If the cooling rate is faster than the rate of mergers, then the vortices fall into a crystal pattern before they can merge. We estimate the merger rate from the observed early-time power law decay of the number of vortices,^2 and estimate the cooling rate from the mixing of the background by the strong vortices. We find that N \propto (N_i)^\gamma, where 0 < \gamma < 1 is an exponent that depends on flow parameters, and N_i is the initial number of strong vortices. This scaling agrees with the experiments as well as with simulations, with no adjustable parameters.

[DP01.151] Shape Oscillations of a Drop in the Film Boiling Regime

A model problem on the capillary-gravity oscillations of a vapour-film levitated drop is considered. Such a problem arises for the shape oscillations of a liquid drop in the film boiling regime. These oscillations one can easily observe if a water drop is in the small curvature spherical cup of the red-hot metal plate. In equilibrium a not very small levitated drop has the form of a rounded cylinder. The height of such a drop h is of the order of a capillary constant a, and the drop base radius R exceeds a. For a free drop in the form of the cylinder h=1.09 a and R=0.54*(V/a)^1/2 , where V is the drop volume. A vapour-film levitated drop demonstrates a great variety of oscillative and convective motions. All the motions take place under the constant feed of energy. The mechanism of arising these motions is open to question. We are interested in the oscillating regime, when the drop takes the form of n-pointed stars (n = 2, 3, 4,...). Visually an oscillating drop looks like 2n-petal rosette. We consider potential motion of drop liquid and take into account gravitational and surface tension forces. We find shape-oscillation capillary-gravity n-modes of a free cylindrical drop. The drop volume dependences of drop sizes (h and R) and n-mode frequencies give qualitative description of experimentally observable patterns.

[DP01.152] Behavior near the superfluid transition in ^4He observed using the MARI spectrometer

We present inelastic (tof) neutron scattering data on liquid ^4He for temperatures around the superfluid transition, obtained using the MARI spectrometer at ISIS and covering a wide range in energy and momentum transfer (-5<E< 40 meV; 0.1<Q<6 ÅAs the temperature is increased from the superfluid state to the normal fluid state, we observe the following: 1) a rapid decrease of the lifetime of the elementary excitations near the superfluid transition, 2) the disappearance of the multiphonon component, 3) the disappearance of the sharp peaks near the two roton excitation energy, and 4) the disappearance of the hybridization between the free-particle and phonon-roton dispersion relations. We show that all of the above observations can be qualitatively described by considering the consequences of the pronounced two-roton resonance in the quasiparticle self energy, and how this resonance disappears as the temperature is increased because of the decreasing lifetime of the roton itself.

[DP01.153] Low Spin State of Fe^3+ in Sr_2FeWO_6

The Sr_2FeWO_6 is a double perovskite oxides with 3d and 4d or 5d tr ansition metals. We performed the X-ray diffraction, Jossbauer absorption spectroscopy, electric resistivity, and magnetic susceptibility measuremen t. The Sr_2FeWO_6 is an insulator with an antiferromagenetic phase t ransition temperarure of 37K. From the Jossbauer absorption spectroscop y of ^57Fe the quadrupole splitting shows large temperature dependence, so the electronic contribution to the quadrupole splitting is large in the s ample. While the temperature dependence of isomer shift is constant. The ext rapolated hyperfine field at 0K is 110kOe, which shows that the Sr_2FeWO _6 has the low spin of Fe^3+. Farther the lattice volume is large vo lume compared with other iron double perovskite oxides Sr_2FeMO_6 (M =4d or 5d transition metals ). The Sr_2FeWO_6 lattice has a bigger d istortion than other iron perovskite oxides. This distortion may be caused b y the Jahn-Teller effect. The electronic state instability between trivalent low and high spin state may lead to the lattice distortion. The Jossbauer results indicate the Fe state in Sr_2FeWO_6 has trivalent valence and the low spin state configuration.

[DP01.154] Diluted Ising Model For The Fe-Al Alloys

The Fe_pAl_q alloys (with p+q=1) in the structural disordered phase have been experimentally studied according to a variety of techniques for a wide range of the aluminium concentration q. It has been shown that the Al atom has no magnetic moment and that the ferromagnetic transition temperature (T_c) decreases as the Al concentration (q) increases. This fact shows that the Al plays the role of a site-dilution in the system. The decreasing in T_c for q < 0.2 is very slow and one has a small value for \alpha = -1/T_c(dT_c/dq) at q=0. However, for q > 0.3 the critical temperature falls down rather abruptly and \alpha is definitely different from zero. Earlier theoretical results, based also on Ising models, were not able to explain this rather unexpected behavior for small concentrations q. Indeed, exact theoretical results on two-dimensional lattices, as well as reliable approximations in three-dimensions, give values of \alpha \sim 1. In this work we propose a simple site-diluted Ising model in which the Al atoms induce a super-exchange ferromagnetic interaction between second-neighbors Fe atoms. It is also assumed that this super-exchange interaction, as well as the nearest-neighbor one, decreases as q increases, since the lattice expands when Fe is substituted by Al. We have used a variational approach based on Bogoliubov inequality for the free energy associated to a kind of pair approximation. Such procedure allows one to obtain closed form expressions for the critical temperature and the phase diagram is easily obtained as a function of the theoretical parameters. The present approach, although rather simple, is better than the usual mean field approximation. In this way, good fittings to the experimental results are now obtained for small q. Moreover, for large concentration of Al, a possible negative value of the super-exchange can theoretically drive an spin-glass like phase in these compounds, an evidence that have already been seen experimentally.

[DP01.155] Self - averaging of random and thermally disordered diluted Ising systems

Randomly diluted three dimensional Ising systems have been studied previously by means of Monte Carlo simulations of the self - averaging (or lack of it) of singular thermodinamic quantities at critically^1,2. In real systems the dilution of vacancies may be either purely random or clustered. We compare Monte Carlo simulations of the critical magnetization in randomly and thermally diluted Ising lattices. Improved self averaging is obtained for critically clustered (critically thermally diluted) vacancy distributions compared with the observed self - averaging for purely random distributions. In highly disordered (marginally clustered) vacancy distributions, the self - averaging is comparable in both cases for the same dilution (p).

1. S. Wiseman and E. Domany, Phys. Rev. Lett, 81, 22 (1998). 2. A. Aharony, A. Brooks Harris ans S. Wiseman, Phys. Rev. Lett, 81, 252 (1998).

[DP01.156] EPR Investigation of CMR Manganite Thin Films

We have measured the EPR spectrum of CMR manganite thin films. We observed an asymmetric absorption line that depends on sample temperature and orientation. We will discuss these results in terms of ferromagnetic resonance and microwave penetration depth.

[DP01.157] Effect Of Anisotropy On The Spin Glass Transition In Diluted Magnetic Semiconductors

Spin glass transition temperatures have been carefully measured for samples of the diluted magnetic semiconductors Cd_1-xMn_xSe and Cd_1-xMn_xS, using zero-field-cooled and field-cooled magnetization and ac susceptibility methods. The electron paramagnetic resonance (EPR) linewidth as a function of temperature was also measured for the same samples. The infinite-temperature linewidths were extracted from the temperature-dependent EPR linewidth data by extrapolation. The anisotropic exchange strengths were then calculated from the infinite-temperature linewidths. From theoretical considerations, the spin glass transition temperature is expected to exhibit a power law dependence on the ratio of the anisotropic and isotropic exchange strengths, with the exponent being 0.25. The experimental data are discussed in the context of this theory.

[DP01.158] \mboxInelastic Neutron Scattering in YbInCu_4 and YbMgCu_4: Testing the Anderson Impurity Model

We report inelastic neutron scattering measurements on YbXCu_4 for X = In and Mg performed on the LRMECS time-of -flight spectrometer at IPNS. These are strongly mixed valent compounds with large Kondo temperatures (\sim500K). We pay considerable attention to correct subtraction of the nonmagnetic scattering (phonon plus multiple scattering). For YbInCu_4, there are two independent ways to accomplish this, and both ways give the same result for the magnetic scattering (J.M. Lawrence, R. Osborn, J.L. Sarrao and Z. Fisk, Phys. Rev. B, to be published), namely that the power function is better fit (\chi2 = 2) by a simple inelastic Lorentzian (E_0\approx 40 meV and \Gamma = 13.5 meV) with \Gamma/E_0 = 0.34 than by the predictions of Anderson impurity model (\chi2 = 10) for which (given n_f(0) = 0.82, where n_f is the Yb 4f hole occuptaion number) the ratio of \Gamma/E_0 should be 0.55. For the case of YbMgCu_4, subtraction of the nonmagnetic background has greater uncertainty, but again, the results are in better agreement with a simple Lorentzian (E_0 = 42 meV and \Gamma = 32 meV) with \Gamma/E_0 = 0.76 than with the predictions of the Anderson impurity model, for which (given n_f (0) = 0.64) the ratio \Gamma/E_0 should be 0.45 . We will also present results for the temperature dependence of the lineshape.

[DP01.159] NMR evidence for slow-moving CDW's structures in URu_2Si_2

We present ^29Si-NMR relaxation and linewidth data which indicate the presence of slow-moving inhomogeneous structures in URu_2Si_2 below about 100~Kelvin. The inhomogeneous broadening saturates near and above T_N\sim17~K, and remains saturated at lower temperatures. A partial structural distortion can be inferred from the behavior of the excess linewidth and excess paramagnetic shift as functions of T. These results indicate that a minority inhomogeneous phase exists below T_N. Measurements of the spin-echo decay rate at low temperatures suggest the structures behave similarly to CDW's in CDW-systems.

[DP01.160] Disproportionation of Wustite Studied by Mossbauer Spectroscopy

Comprehensive understanding of formation and disproportionation mechanisms of iron oxides is indispensable to control the fabrication processes of various steel products. A wustite (Fe1-xO) phase having homogeneous composition was synthesized from alpha-Fe and Fe3O4 in a vacuum sealed tube at about 1100 K, and the disproportionation process at temperatures below transition temperature (843 K) was studied by Mossbauer spectroscopy. The change of the fraction of components by annealing in a sealed tube depends linearly on the logarithm of the annealing time. The reaction speed governed by two factors, i.e. the difference of free enegies in the reaction equation and the migration rate of Fe ions in the solid phases, was found to have the maximum at around 400 K.

[DP01.161] Magnetic semi-disorder in Fe-doped cupric oxide

Investigations of the abnormal magnetic properties of cupric oxide reveal discrepancies between both experimental results and theoretical explanations. Doping cupric oxide with ^57Fe we have been able to obtain Mössbauer results that are an experimental evidence of magnetic semi-disorder. Two transitions are observed, one at about 150 K, that can be assigned to the long-range ordering of the cupric oxide spins, and a second one at some temperature between 4.2 and 15 K, that exposes either the freezing of the Fe^3+ spins into a local canted state or of magnetic clusters in the CuO matrix.

[DP01.162] Renormalization-Group Treatment of Quantum spin-1 Chain

The RSRG approach introduced by Ma, Dasgupta, and Hu (MDH) has proved to be the most successful to explain the low temperature properties of the spin-1/2 random antiferromagnetic chain (RAFC). The system flows toward a fixed point with universal thermodynbamic properties and power law behavior, a phase which has since been called random singlet phase (RSP). A straightforward application of that method to a spin-1 system does not predict the existence of a Haldane phase --which is expected to exist at least in the weak disorder regime. In the present study we use a generalization of MDH scheme to investigate the nature of the RSP of the disordered spin-1 Heisenberg chain. Our results predict a phase transition from the random singlet phase to a Haldane phase.

[DP01.163] Anisotropic DC Penetration Depth and Magnetic Properties of UPt_3 Superconducting Phases

In order to identify the properties of the UPt_3 superconducting phases we have studied the dc penetration depth with a SQUID magnetometer and the anisotropic magnetic field response with a capacitive torque meter. The samples were four single crystals grown with different methods and annealed differently. When \vec B is in the c-direction we find a clear s.c. double transition in \lambda (T) while for \vec B\|a only a single transition is observed. This result is in accordance with 2-dimensional models of the s.c.\ order parameter, but not with 1-d and weak spin-orbit coupling scenarios. Below 20~mK a steep "diamagnetic" drop occurs, coinciding with the specific heat anomaly which we found earlier at 18 mK. This seems to indicate long-range AF order. The torque meter magnetization curves in the s.c.\ regime show strong irreversibilities up to a line of fields parallel to the B_c2(T) curve. In the cleanest sample this line is shifted to fields below the B-C internal phase line which then manifests itself in a pronounced kink of the magnetization curve leading to an enhanced upper critical field, a clear evidence for the unconventional character of this transition. We also observe an anomalous peak effect, a region of enhanced flux pinning near B_c2, characteristic for the Fulde-Ferrell-Larkin-Ovchinnikov state.

[DP01.164] Domain Wall Dynamics in KDP-Type Ferroelectrics

Polarization switching in the ferroelectric KDP is attributed to domain wall motion under the influence of an external field, with the wall mobility influenced by the energy barrier for the formation of ions (Takagi groups) ahead of the wall, as well as their drift in and near the wall. Polarization fluctuations can also result from the motion of Takagi groups that displace the domain wall. Formation of these ion groups is a stochastic process and the wall can be modeled as a strongly coupled set of multi-stable oscillators. Such a model is used to explore the microscopic dynamics of a domain wall in a H-bonded KDP-type ferroelectric with emphasis on the (possibly nonlinear) interaction with the thermal bath and additional applied external noise. Local cooperative motion results in dynamic wall roughening on a wide range of scales which may be detectable in spectral analysis. We also investigate the possibility that additional noise might enhance domain wall mobility and thus increase switching speed.

This work is supported by NSF Grant DMR-9805272.

[DP01.165] Relaxor Properties and Domain Structure of SBN:Ce

Relaxor properties of Sr_0.61Ba_0.39NbO_3:Ce (SBN:Ce) are investigated by linear birefringence (LB) and dielectric spectroscopy. LB measurements show a random-field induced tail, which extends to more than 100K above T_c. Dielectric spectroscopy reveals that the polydispersivity increases with increasing Ce concentration. Surface corrugations due to volume holographic recording are examined by atomic force microscopy, from which the local field is evaluated. After cooling the crystals to room temperature (RT) from above T_c, cylindrical domains on a micrometer scale are observed. Very probably they originate from the nanodomain state, which freezes in at T>RT.

[DP01.166] Aging and off-equilibrium slow dynamics in a lanthanum-modified lead zirconate titanate relaxor system

The history-dependent effects such as aging have been studied by measurements of the frequency-dependent complex dielectric constant in a 9/65/35 lanthanum lead zirconate-titanate (PLZT) ceramics. It was found that the aging process strongly influences both the static dielectric constant and the dielectric polydispersivity. Specifically, the aging process is responsible for the difference in temperature variations of the field-cooled static dielectric constant and the static dielectric constant determined in the dynamic zero-field-cooled experiment. Analysis of the complex dielectric susceptibility has revealed that for an aged PLZT sample the ergodicity of the relaxor phase is effectively broken due to the divergence of the longest relaxation time in the vicinity of 250 K. The electric-field--temperature phase diagram was established by dielectric measurements. It is shown that by cooling the PLZT ceramics in a dc electric field higher than 5 kV/cm, a long-range ferroelectric phase is formed.

[DP01.167] Soft Modes and Phase Instability of the Low-Temperature Phase of Ba_xSr_1-xTiO_3

Raman scattering spectra of the low-temperature rhombohedral phase of Ba_xSr_1-xTiO_3 solid solutions have been studied at T=6^\circK for the first time. All modes of the rhombohedral phase were identified using polarized Raman scattering spectra of BaTiO_3 and Ba_0.12Sr_0.88TiO_3 single crystals. This data allowed us to identify the modes and the symmetry of vibrations in the Raman spectra of Ba_xSr_1-xTiO_3 ceramics. In the spectra of Ba_xSr_1-xTiO_3, soft modes of Ti-O bond vibrations were found to be dependent on Ba concentration. The frequencies of soft modes decrease from 270 cm^-1 (E-mode) and 180 cm^-1 (A-mode) at x=1 in pure BaTiO_3 to zero at x=0.025. This means that the ferroelectric rhombohedral phase is unstable at x=0.025 and it transforms to the nonpolar tetragonal phase of SrTiO_3. It is important to note that the lattice constant of SrTiO_3 is smaller than that of BaTiO_3. Therefore at decreasing lattice constant of Ba_xSr_1-xTiO_3, the stiffness of Ti-O bond falls steeply but does not grow, as it usually does. This result validates the correctness of a multiwell model of Ti-O potential, and supports the concept of order-disorder phase transitions in BaTiO_3.

[DP01.168] Micro-gravity CDOT-II experiment results

An extension of previous experiments on the hard sphere phase diagram and crystal morphalogy was performed in \mu g on space shuttle Discover STS-95 flight. All coexistence samples exhibited dendritic growth and all ``glass" samples crystallized in two days. We report on the behavior of ``glass" sample crystallized on orbit and partially melted on erath.

[DP01.169] Texture and dielectric properties of Ti doped LSBN-SBT complex ceramics

The stoichiometric LSBN-SBT ceramic system was obtained for different concentrations of titanium ions (Ti(%)=1,3,5,10,30,50), where the Nb^5+was substituted by Ti^4+ according to the following expression Sr_0.255La_0.03Ba_0.7Nb_2-yTi_yO_6-y/2. The oxygen vacancies thus created which, together with the vacancies in the A sites of the TTB material structure, have an important influence in their texture and dielectric properties, due to the crucial role of the occupancy of the crystallographic sites. Evidece of a crystallographic phase transition is observed for y=0.1. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and thermoelectric analyses are reported.

[DP01.170] Critical behavior of inhomogeneous systems described by a non-local correlated random field

We consider the critical behavior of d-dimensional inhomogeneous systems with a frozen-in random field having a non-local correlation function decaying as a power law x^-b. The problem is studied within the context of an exact model which considers interactions of fluctuations partially. A new critical behavior arises which is strictly due to the long-range nature of the correlation function of the random field as long as b is less than d. In that case b is playing the role of an effective dimensionality. Then, first, when b\leq 4, the system is always in the disordered phase. An ordered phase occurs for b>4. For this case a d-dimensional long-range randomly correlated system behaves as one of effective dimensionality b and has the same critical behavior as a pure d^\prime -dimensional system where d^\prime =b-2. If b=6, the system has a logarithmic behavior, a result strictly due to the long-range correlations, despite the fact that for dimensionality greater than 6, a system with short-range interactions only, had the usual mean-field behavior. The mean-field behavior is restored only if both b and d are greater than 6. For b=d the behavior is qualitatively the same as in the case without the long-range term, with both the short and long-range terms contributing equally and indistinguishably. Upon suppression of fluctuations, all results reduce to those of mean field theory, regardless of dimensionality or the presence of impurities.

[DP01.171] Finite-Size Effects in Antiferromagnetic Heisenberg-Ising Chains

The spin 1/2 Heisenberg-Ising chain, with the axial anisotropy as a parameter, can be exactly solved through the Bethe's Ansatz method. Based on saddle point asymptotic expansions of the Bethe's equations, de Vega and Woynarovich proposed a systematic procedure to compute finite size corrections for large but finite chains (H.J. de Vega and F. Woynarovich, Nuclear Physics B 251), 439 (1985). In our work, we use the Ansatz equations to numerically solve finite-size systems for chains up to thousand sites. This way, we study size effects on the energy levels directly from numerical data. In particular, we obtain scaling laws with size for the ground state, the mass gap and the spin wave states, that enlighten our understanding of the crossover regime into the asymptotic behavior predicted by the analytic calculation mentioned above. We note that this information is difficult to be obtained by other methods, specially due to the large sizes that can be attained using the Bethe Ansatz. Finite size effects are used to estimate the correlation length following different schemes that are compared with results from the exact 6-vertex model. Finally, we critically comment on extrapolation procedures near the crossover regime.

[DP01.172] Orientational order in adsorbed liquid layers

A fluid layer can be rendered anisotropic by contact with an anisotropic medium.The effect of an orientation dependant substrate potential is studied for adsorbed linear molecules with weak anisotropy . It is shown that orientational order can be induced by surface interactions near the freezing temperature of the fluid even for liquids which do not show liquid crystal phases.The variation of the density and the orientational order parameter near the surface and as a function of temperature is given. An unusual negative temperature slope of the interfacial energy results.

[DP01.173] Scaling in Relaxation of Relaxor Ferroelectrics

We have measured the dielectic susceptibility of 0.9PbZn_1/3Nb_2/3O_3-0.1PbTiO_3 at 5 decades of frequency. We analyze the temperature dependent relaxation time distribution and the dynamical behavior. The data show two different relaxation mechanisms above and below spontaneous relaxor-normal ferroelectric transition.The temperature dependence of characteristic relaxation time in the high temperature region exhibits typical relaxor behavior like PbMg_1/3Nb_2/3O_3. The relaxation behavior at low temperature follows Arrehenius law and can be described by a single Debye relaxor. The scaling relationship reported by Nagel and co-workers for a variety of structural glasses is re-examined. The analysis shows that dielectric loss with two different relaxations falls onto a single master curve.

[DP01.174] Theoretical study of isotope effect for K_3(H,D)(SO_4)_2 by direct determination of both electronic and protonic (deuteronic) wave functions

The phase transition temperature of hydrogen-bonded ferroelectric materials drastically changes upon replacing hydrogen atoms with deuterium atoms. Although there are many models for the isotope effect on the phase transition (proton tunneling model, geometrical isotope effect, etc), the origin of the isotope effect is still unknown.

In order to explore the possible origin of the isotope effect theoretically, the recently proposed first-principle dynamical extended molecular orbital (DEMO) method is applied to K_3H(SO_4)_2 (KHS) and K_3D(SO_4)_2 (DKHS). Since the DEMO method determines both electronic and nuclear wave functions simultaneously by optimizing all parameters including basis sets and their centers variationally, we obtain different nuclear orbitals for the proton and deuteron as well as their electronic wave functions. The calculated hydrogen-bonding distance in DKHS is larger than that in KHS. This indicates that the deuteron has weaker hydrogen-bonding than the proton (Ubbelohde effect). Since the electronic and protonic (deuteronic) coupling effect is directly taken into account by the DEMO method, the istope effects on structures, wave functions, and energies between KHS and DKHS are clearly demonstrated.

[DP01.175] High Pressure X-Ray Diffraction Study of CoO to 99 GPa

We have studied phase transition and equation of state of CoO to 99 GPa at ambient temperature. Polycrystalline CoO samples were compressed in Mao-Bell type diamond cells and probed with energy dispersive x-ray diffraction (EDXD) at the superconducting wiggler beamline X17C of National Synchrotron Light Source. We discovered that cobalt oxide transformed from the face-centered cubic B1 structure to a rhombohedral structure bellow 66 GPa under nonhydrostatic conditions. The transition is reversible. At the transformation there is no sudden volume drop. The P-V data fitted with Birch Murnaghan equation. That yields the bulk modules of CoO at zero pressure is 212 GPa

[DP01.176] X-ray studies of BeO under high pressure

BeO is an interesting ceramic because of its electrical, thermal and crystal properties. The ambient crystal structure of BeO (B4) differs from the other BeX (X=S, Se and Te) chalcogenides (B3). X-ray diffraction studies of BeO under high pressure were carried out to 126 GPa by using energy dispersive x-ray diffraction in CHESS. To enhance the strength of the gasket and to increase the thickness of the sample at high pressure, we used an annulus of diamond-epoxy around the sample. As a result, excellent patterns were obtained even at pressure above 100 GPa on this low Z material. The B4 structure remains stable above 100 GPa, which is different from other BeX. The bulk modulus of BeO was determined 265\pm22 GPa by fitting with the Birch equation. The goal is to find the transition pressure to a more highly coordinated structure and to compare this with theory (P. E. Van Camp and V. E. Van Doren, J. Phys.:Condens. Matter 8,3385(1996).).

[DP01.177] Research at Universities: Ivory Tower or Competitive Enterprise ?

There is considerable controversy concerning how research performance can be measured. Another topic of current interest is the relative merits of business firms versus university research establishments. Here, we analyze academic research and compare its growth dynamics with that of business firms. We analyze a National Science Foundation (NSF) database of the research expenditures (budgets) for science and engineering of all major United States (US) universities for the 17-year period 1979--1995. We find that the dynamics of the growth process, measured by the growth rate in research budgets follow scaling laws. Remarkably, these scaling laws are the same as that observed for firms(M.H.R.\ Stanley et al.), Nature 379, 804 (1996) and for countries(Y.\ Lee et al.), Phys. Rev. Lett. 81, 3275 (1998) . Further, we analyze the research budgets(size) of schools comprising universities. We find that the distribution of research budgets of schools within a university of a given budget, has the same functional form for universities of different scales of research budgets; the spread of this distribution, increases with university size as a power law. Furthermore, we find that the same results hold for the departments within a school, which suggests a scale-free structure of organization for universities which may also be present for firms and countries.

[DP01.178] Self-Assembled Crystals of Passivated Ag Nanocrystals

Three dimensional crystals composed of Ag nanocrystals of approximately 4.5nm diameter passivated with dodecanethiol self-assembled-monolayers have been studied via Transmission Electron Microscopy (TEM), Electron Diffraction (ED) and Atomic Force Microscopy (AFM). The two dominant periodic nanocrystal packing structures found are hexagonal-close-packed (HCP) and face-centered-cubic.(FCC). The FCC packing structures have lengths on the order of tenths of microns while the HCP structures have very uniform platelet-like morphologies of approximately 100nm thick and up to 3 microns across with well defined superlattice facets along the three lowest ordered planes of the [0001] zone axis forming triangular shapes. In-situ high temperature TEM experiments show the HCP structures to be stable up to 975K. The unusually strong inter-nanocrystal bonding is believed to be due to interpenetration of the thiol surfactant amongst neighboring nanocrystal cores, this confirmed by Enery Filtering TEM.

[DP01.179] Oscillatory Chemical Kinetics without Deterministic Feedback

Most models for chemical oscillations use at least three active degrees of freedom and postulate a deterministic feedback mechanism to produce the oscillations. We have performed computer simulations of a model with no such deterministic feedback process that also shows oscillations in reaction rate. The model mimics CO oxidation on a catalytic surface and includes a stochastic process where the vacant adsorption sites can randomly flip from accepting adsorbates to an inert state where adsorption is forbidden. The oscillatory process proceeds by nucleation and growth of active regions that in some regimes resemble ``target patterns'' characteristic of deterministic models. A mean field analysis following the density of reactants and an improved analysis based on studying nearest neighbor pairs is developed and compared with the simulations.

[DP01.180] Isotopic patterns as quantum automata for self-organization

Isotopic permutations in crystalline structures have information storage capacity N=10E21 bits/cc. Isotopic diversity makes entire crystal a single informationally connected entity [1]. Since isotopic patterns are energetically quasi-degenerate, high coherency and informationally enhanced neutron tunneling [1] opens possibility of entire crystal acting as a multiple state quantum computer (quantum automata of David Albert) with number of qubits of N! order. Backward causality of Deutsch-Stapp type translates to possibility of isotopically (quasi)random system acting as receptor of potential patterns along path integral tenet that everything that could happen tries to [2] and it is resulting interference patterns, both constructive and destructive, that determine the reality. Virtual Babylonian library (J.L.Borges) of all possible patterns for self-organization contrives Cantor-countable set which can be Godel- numbered through prime numbers. The latter points towards possible essential role of superlong primes of T(4) and higher order of tower-exponentiation [2, 3]. References: [1] A.A.Berezin, J.Theor.Biol. 154, 415, 1992; [2] A.A.Berezin, URAM Journal, 21, 122, 1998; [3] A.Abel, Saturday Night (Toronto), November 1998, p. 38-39.

[DP01.181] Random laser action in disordered media

We report experimental evidence of laser action in strongly scattering semiconductor powder and polycrystalline films. Multiple optical scattering leads to the formation of closed loop paths for light, and these loops could serve as ring cavities for lasers. The characteristics of lasing in random media exhibits remarkable differences from that of a conventional laser. Our observation may open up the possibility of utilizing disordered semiconductor microstructures as alternative sources of coherent light emission.

[DP01.182] Model and properties of an electronic relaxation oscillator with voltage protection

In an electronic relaxation oscillator the voltage across a capacitor relaxes between a sine-signal modulated upper threshold and a lower threshold modulated by the phase of the signal at the point where the relaxation oscillation reaches the upper threshold. When the relaxation voltage across a constant value E, a protection mechanism is equipped in the circuit so that the modulation sine-signal can be replaced by E. From the differential equations describing the circuit we have deduced its 2-d Poincare mapping. It is interesting that the map is an area-preserving one and its properties relating to the transitions from regular to fully chaotic motions are quite unique. The details of our observation will be published soon.

[DP01.183] Nonlinear noise spectra in one and two dimensions

We present a general theoretical formulation, based on nonequilibrium Green's functions, for nonlinear noise spectra in multi-probe mesoscopic conductors. The theory is gauge invariant and is useful for the predictions of noise spectra at finite voltages. At small voltages, we have derived the expression for the second order nonlinear noise spectra. In the wideband limit, the exact solution of the noise spectra for finite voltages has been obtained. We have also calculated numerically the second order nonlinear noise spectra for a two-dimensional mesoscopic conductor.

[DP01.184] Predicting Earthquakes from Sand Piles Using Self-Organized Criticality

It has been proposed [J. Rosendahl et.al, Phys. Rev. Lett. 73, 537 (1994)] that earthquakes can be modeled by avalanches in a simple sand pile system and that increased activity before a major avalanche could be used to predict the onset of major earthquakes. With smooth glass beads forming a conical pile, we find experimentally that adding one bead at a time causes many small, and a few large, avalanches consistent with the predictions of Self-Organized Criticality. In particular, the number of avalanches of a given size is proportional to the size of the avalanche to the power (-1.48\pm0.05). We do not see any increased activity before a major avalanche, which is consistent with the behavior of earthquakes associated with some faults, but inconsistent with Rosendahl's results and the behavior of other faults. We also looked at foreshocks, aftershocks, and the average time between major avalanches. This research is supported in part by REU grant NSF-DMR 9619406.

[DP01.185] Fractal Conductance Fluctuations in Mesoscopic Billiards: an Observation Over Three orders of Magnitude

Fractal magneto-transport properties of mesoscopic semiconductor billiards is highly topical [1-4]. In these studies, the magnetic field range over which fractal behaviour can be observed is crucial. Previous observations have been limited to approximately one order of magnitude. We present fractal conductance fluctuations observed over three orders of magnitude and discuss the physical conditions required to extend this range.

[1] R. Ketzmerick, Phys. Rev B, 54, 10841 (1996). [2] R.P. Taylor et al., Phys. Rev. Lett., 78 1952 (1997). [3] A.P. Micolich et al., J. Phys. Condens. Matt. 10, 1339 (1998). [4] A.S. Sachrajda et al., Phys. Rev. Lett., 80 1949 (1998).

[DP01.186] Generalization of the Fluctuation-Dissipation Theorem

We consider a system of interacting bosons and fermions and derive a system of equations, which generalizes the fluctuation-dissipation theorem to the case of non-linear systems.

[DP01.187] A crisis in a relaxation oscillator

We have observed a kind of crisis in a 1-d piece-wise smooth mapping that describes an electronic relaxation oscillator. The crisis happens via a collision of an unstable periodic orbit positioning on the basin boundary of a chaotic attractor with the discontinuous set of the mapping. Analytically we have found that the averaged characteristic time varies as an exponential function of the driving parameter with an exponent -1.8. This special exponent can be explained by the fact that the size of the "leaking hole" formed after the crisis is an exponential function with an exponent 1.1, and that the averaged visiting probability in the hole is also an exponential function with an exponent 0.7.

[DP01.188] A Friction Driven Oscillator

A rigid rod is supported horizontally by two stainless steel wheels that counter-rotate at the same speed. The friction forces exerted on the rod at the wheels are in opposition to each other and the excursions of the rod are thus globally confined. We analyze the times series of the position of the rod over long times. We find that the random nature of friction manifests itself in modifying the back-and-forth oscillation of the rod. It evolves from a harmonic oscillator at high speeds, to a Brownian-like particle behavior at low speeds.

[DP01.189] Phase space structure and pattern formation

The equations of motion of incompressible fluids and magneto-fluids are essentially nonlinear, although they contain linear dissipative terms. Omitting these dissipative terms creates systems of nonlinear equations in which total energy and other integrals are conserved. These `ideal' systems may serve to model some of the aspects of real turbulent fluids (although they should never be confused with them), or of other dynamical systems. Finite Fourier representations of these model systems have an intriguing statistical behavior, an apparent nonergodicity which has been observed in computer simulations. This behavior can be explained by the presence of certain pseudo-scalar integrals called `helicities', which allow the definition of set-theoretic characteristic functions. These functions partition the phase space spanned by the independent Fourier coefficients into disjoint components. This disjointness gives rise to the formation of patterns in the `turbulent flows' of these conservative dyamical systems, since the phase point is constrained to have a non-zero time average.

[DP01.190] Statistics of Wave Dynamics in Random Media

We measure the joint distribution of the microwave transmission coefficient through a random medium \epsilon_ab and the single channel delay time \tau_ab, which in the limit of zero pulse bandwidth is the frequency derivative of the phase of the transmitted field. For fixed \epsilon_ab the distribution of \tau_ab is a Gaussian with variance inversely proportional to \epsilon_ab. In contradistinction to \tau_ab, the dynamic matrix element \epsilon_ab\tau_ab has substantial long-range correlation with frequency shift. The results are compared to a new dynamic theory for a Gaussian process, which is in excellent agreement with experiment.

This work is supported by NSF grant Nos. DMR9632789 and INT9512975 and the Groupement de Recherches POAN.

[DP01.191] Monte Carlo Simulation Study of the Standard Three-Dimensional Driven Diffusive System

We study by extensive Monte Carlo simulation a lattice gas model driven by an external field with a periodic boundary condition, known as driven diffusive system. We analyze the critical behavior of the standard three-dimensional model. Both anisotropic finite-size scaling based on field-theoretic results and empirical isotropic scaling are considered.

[DP01.192] Scaling Laws for Noise Enhanced Propagation

In the phenomenon of stochastic resonance, a nonzero value of noise optimizes the response of a nonlinear system to a deterministic signal. Noise enhanced propagation is a recent, generic, and nontrivial extension of stochastic resonance, in which noise extends signal propagation in spatially extended nonlinear systems. In a simple numerical model, we sinusoidally force one end of a chain of overdamped bistable oscillators with nearest-neighbor two-way coupling. In the presence of noise, we record a signal-to-noise ratio at each oscillator. We demonstrate that moderate noise significantly extends the propagation of the sinusoidal input, while too little or too much noise does not. Oscillators far from the input-in the region of the chain where noise extends the signal-exhibit a classical stochastic resonance. We study the scaling of the optimal noise and maximum propagation length with the shape of the bistable well and with the coupling. The simplicity of the model suggests the generality of the phenomenon. This research was supported by NSF grant DMR 9619406 and The College of Wooster.

[DP01.193] Chaotic Brownian Billiards

We generalize the classic problem of a billiard bouncing inside box by adding a movable disk to the box. When the billiard collides with the disk, the disk recoils so as to conserve energy and momentum, in a way reminiscent of Brownian motion. The square box "wraps" the disk, with periodic boundary conditions, so that it effectively moves on a torus. The box either reflects the billiard or wraps it. In each case, we numerically estimate motion and Lyapunov exponents. When the box reflects the billiard, the disk diffuses such that its distance traveled is proportional to the square root of the elapsed time. When the box wraps the billiard, the disk "streams" such that its distance traveled is proportional to the time. The dramatic difference in these motion exponents reflects whether or not the combined momentum of the disk and billiard is conserved. In either case, positive Lyapunov exponents reflect the chaotic motion of the disk. We obtain similar results for multiple billiards. This research was supported by NSF grant DMR 9619406 and The College of Wooster.

[DP01.194] Taming Chaos in an Array of Pendulums

We designed and constructed an array of ten forced damped nonlinear pendulums. We drove the pivot of the pendulums in a circle and torsionally coupled them with springs. We analyzed the motion using digitized videotape. The behavior of the real array closely mirrored the behavior of its computer simulation. For a homogeneous array of identical pendulums, the spatiotemporal dynamics was chaotic; for a heterogeneous array of nonidentical pendulums, the spatiotemporal dynamics was periodic: Disorder had tamed chaos. This research was supported by NSF grant DMR 9619406 and The College of Wooster.

[DP01.195] Viscous Fingering in Shear Thinning Fluids

We have carried out viscous fingering experiments in radial Hele-Shaw cells containing shear thinning fluids. We have observed the growth of stable fingers, resembling dendrites, as has been predicted by earlier numerical simulations (L Kondic, M. J. Shelley, P. Palffy-Muhoray, Phys. Rev. Lett. 80) 7 (1998). These results indicate that dendritic growth can occur not only in anisotropic systems, but also in isotropic systems with non-linear transport coefficients.

[DP01.196] A dynamic model for diamond film growth

We propose a dynamic model to describe the process in which activated hydrocarben species are transmitted to a substrate to grow a diamond film. The reactions between the species, the adsorption and desorption of them on or from the surface of the substrate, the surface reaction in the growing process, and the influences between these factors are considered in the model. The parameter region in which diamond film grows has been computed after solving the differential equations of the model. The results show a very good agreement with the experimental results obtained by us and other people.

[DP01.197] Molecular Weight and Time Effects in Adhesion

We have investigated adhesion and adhesion hysteresis using different molecular weights of crosslinked poly(dimethyl siloxane) (PDMS) hemispherical lenses and silica surfaces (SiO2 on Si), using the JKR (Johnson, Kendall, and Roberts) method. Three different experimental protocols were used: a stepwise mode (5 min waiting between data points to reach a quasi-equilibrium state) a continuous mode without waiting between loading and unloading, and a continuous mode with 30 min intermediate waiting. An increase in adhesion strength was observed with increasing molecular weight. It was found that a simple power law describes the dependence of G from the molecular weight. Interestingly, the exponent is different for each modes of the experiment. The exponent for each case was found to be 2/3(stepwise), 1/2(continuous without waiting), and 1/3 (continuous with waiting). It is noteworthy that for the continuous mode without waiting gives ~1/2 which was predicted by deGennes. We are currently investigating the cause of this difference. Preliminary results suggest that relaxation processes in the polymer network may be one of the main reasons.

[DP01.198] Size effect in the dynamics of early droplet growth

Experiments in many systems from water condensation to metal deposition have demonstrated the existence of a crossover in the growth law of the radius of a droplet during nucleation on a substrate.A theory for growth of a droplet during a first order phase transition is presented. In the early stage, growth occurs through particle diffusion inhibited by the surface constraint of the spherical droplet. As the droplet size increases, the surface curvature decreases and a transition to rapid field free propagation of the concentration front occurs.Results are given for the initial and asymptotic droplet distribution and for the droplet growth law.

[DP01.199] Gas Phase Radical Column Densities in a Diamond Growth Flame

Highly sensitive absorption spectroscopy is used to measure column densities of various radicals during the flame deposition of diamond. Column densities of C_2, CH, CN, and OH are measured as a function of position and flame mixture in an atmospheric pressure oxyacetylene torch. The gas temperature is derived from the C_2 Swan band spectra. The relative densities of singlet and triplet C_2 are examined, and upper limits of the CH_3 and C_3 column densities are presented. Finally, the measured column densities are correlated to growth quality as judged by Raman spectra and scanning electron microscopy. This work demonstrates the applicability of using multielement detector arrays to obtain absorption spectra even with significant atomic or molecular emission from the absorbing sample.

[DP01.200] A Sign of Turbulent Formation Processes for Gamma-Ray Bursts

As we get closer to a definitive distance scale for gamma-ray bursts (GRB) more attention is being focussed on the physical processes that cause them. In this paper our fractal analyses show scale invariance in GRB time profiles. Furthermore, our Fourier spectrum approach shows that the scaling exponent is consistent with the results of others. This, along with our cycle analysis, are clear signs of the presence of turbulent formation processes at the GRB sites.

[DP01.201] An Analysis of the Morphology of Cluster-Cluster Fractal Aggregates

We have studied the morphology of fractal aggregates resulting from cluster-cluster simulation dynamics. For our analysis, we have studied several models including the diffusion limited cluster aggregation model (DLCA), the hierarchial model, and the restricted hierarchial model. Sorensen and Oh (C.M.Sorensen and C.Oh, Phys. Rev. E58) Divine Proportion Shape Preservation and the Fractal Nature of Cluster-Cluster Aggregates (Dec. 1998) have shown that the fractal dimension is analytically related to the aspect ratio for the restricted model. Here, we study the relation between aspect ratio and the fractal dimension for other models.

[DP01.202] A Complicated Multipal Devil's Staircase

We have observed a more complicated multipal Devil's staircase in a piece- wise linear 1-d mapping which can be viewed as a simplified model of an electronic relaxation oscillator. This mapping contains three discontinuous regions. A periodic orbit will suddenly change to another periodic one after colliding any of them. Thus when a driving parameter changes, a Devil's staircase forms. Each phase-locking step in the staircase is confined by one of the collision conditions. We have deduced all the end points positions of the steps in the driving parameter space. The analytically obtained winding numbers of the steps show a lot of tower-like structures in the winding number -- parameter plane, but the corresponding steps in each tower form a much more complicated structure than what can be found in a ordinary multiple staircase.

[DP01.203] A generalized single wave model for unstable Electrostatic waves

The nonlinear evolution of a unstable electrostatic wave is considered for a multiple-species Vlasov plasma. From the singularity structure of the associated amplitude equation, the asymptotic features of the electric field and distribution functions are studied in the weak growth rate limit. The asymptotic electric field is monochromatic at the wavelength of the linear mode with a nonlinear time-dependence. The structure of the distributions outside the resonant region is given by the linear eigenfunction but in the resonant region the distribution is nonlinear. This generally derived picture corresponds to the single wave model originally proposed by O'Neil, Winfrey and Malmberg for the special case of a cold-beam instability in a plasma of fixed ions. The ``single wave'' reduction of a multi-species Vlasov plasma supporting a weakly unstable electrostatic mode is implemented. The reduced model is a Hamiltonian system and preserves the essential physical features of the full problem. We discretize the model and follow the evolution of an unstable wave numerically; special attention is paid to the scaling of wave amplitude with the linear growth rate.

[DP01.204] Numerical studies of the abelian sandpile model as the relaxation dynamics is changed from "burst-out" to "diffusion-like"

The properties of the abelian sandpile are investigated as the relaxation rule is changed continuously between the original "burst-like" one and one that resembles ordinary diffusion. Our numerical results tend to indicate that the system changes continuously (as far as the values of the critical exponents are concerned) between the two regimes. Some intuitive arguments explaining these observations are also presented.

[DP01.205] A thermodynamic model for the growth of a diamond film including hydrogen

It is possible that during the process of a diamond film growth some hydrogen atoms remain inside as defects. We propose a model to describe a kind of ideal structure of the "diamond layer including hydrogen", the corresponding surface reactions and the growing mechanisms. The possible parameter conditions are solved according to some thermodynamic calculations. The results may show an agreement with some experimental results.

[DP01.206] Mode Interactions of Globally-coupled Phase Oscillators

The onset of collective behavior in a population of N globally-coupled phase oscillators with randomly distributed frequencies is studied when there are competing synchronizing transitions at distinct mode numbers n>l\geq1. In addition to the distribution of native frequencies, additive white noise is included in the phase equations to produce diffusion in the phases. For large N, the population is described by a kinetic equation and the normal form equations are derived by center manifold reduction. The resonant mode interaction with n=2l is considered in detail for populations with couplings and frequency distributions that are reflection symmetric so that the model has O(2) symmetry.

Part D of program listing