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 0.0064¡Ó0.0008, 0.0117¡Ó0.0025, and 0.0083¡Ó0.0005, 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 considere