Session J15 - Poster Session II.
POSTER session, Sunday morning, April 20
Congressional Hall, Renaissance

[J15.01] Spectrum and Energy Levels of Hg II

This abstract was not submitted electronically.

[J15.02] Dielectric Relaxation Studies of Selected Materials Using a Microwave Resonant Cavity as a Probe.

This abstract was not submitted electronically.

[J15.03] Intensity Dependent Absorption and Transit Relaxation on the Cesium D_1 Line

A nonlinear absorption is observed when an atomic transition is saturated. This intensity dependent absorption was measured on the D_1 line in atomic cesium. The spatial cross section of the laser beam and the magnetic field applied to the vapor were controlled and varied during data collection to manipulate the transit relaxation and magnetic sublevel mixing rates, respectively. A three-level rate equation model does not successfully reproduce the data obtained in the absence of a magnetic field. Hence, a more complex and complete model that explicitly includes all of the hyperfine magnetic sublevels (a multilevel model) and accurately models all of the data is presented. The good agreement between this model and the data allows the determination of the transit relaxation rate (due to atomic time of flight through the laser beam), \Gamma_t=(1.13\pm0.20)v_rms/D, where v_rms is the two-dimensional rms speed of the atom and D is the beam FWHM.

[J15.04] Atomic Polarizabilities: Quantal and Classical Perspectives.

The mechanism by which an external electric field induces in an atom an electric dipole moment is formulated from classical and quantal points of view. It is shown that the expressions for the atomic polarizability derived from each of these viewpoints are, to the same level of approximation, identical. The classical perspective is, however, more intuitive and provides a more insightful picture. The results are compared with our recent measurement of the polarizability of the nominally hydrogenic 30d-state of sodium (PRA, 54) R4645 (1996).

[J15.05] Visible Emission Spectrum of Hg-Ga Excimer

A spectroscopic study of Ga-Hg has been carried out for the first time. The Ga-Hg mixture in a sealed tube excited by an rf discharge show new excimer spectra in the visible region, the emission centered around 501 nm. The density of Hg is in the range of 10^18 - 10^20 atoms/cc and that of Ga about 10^11 atoms/cc. Spectra have been studied at a dispersion of 1.2 A^o/mm. Vibrational structures have been resolved and analyzed on the basis of triatomic species. The emitter is most likely a HgGa_2 excimer.

[J15.06] Emission and Predissociation in the O_2^+ b^4\Sigma _g^- - a^4\Pi _u System

The First Negative system of O_2^+ is examined at high resolution using the techniques of fast ion beam-coaxial laser photofragment spectroscopy (FIBLAS) and Fourier Transform emission spectroscopy (FT) to measure an accurate and fully interconnected set of term energies for the first six vibrational levels in the predissociated b^4\Sigma _g^- state relative to the a^4\Pi _u(v=0-7) state. Identification of rotational lines within the complex structure of these quartet bands was facilitated by a Zeman splitting of the emission lines in the FT spectra, arising from a weak magnetic field in the emission source, and by (predissociation) lifetime broadening of the absorption lines measured using the sub-Doppler FIBLAS technique. Line positions in each of the 16 FT bands and the 7 FIBLAS bands were fitted to ^4\Sigma and ^4\Pi Hamiltonians to determine a consistent set of molecular constants describing the a^4\Pi _u(v=0-7) and b^4\Sigma _g^! -(v=0-5) levels with an absolute accuracy of better than 0.02 cm^-1.

[J15.07] Optical Emission Spectroscopy of a 150kW DC Arc Torch: A Comparison of Transferred vs. Non-Transferred Modes

DC arc torches are proposed or in use for solid waste remediation at several sites. However, there is no consensus on the optimal mode of operation: transferred or non-transferred arc. As part of a project to investigate plasma treatment of shipboard waste, we have been investigating both modes at atmospheric pressure. This paper reports on the use of visible optical emission spectroscopy to determine the electron temperature, T_e, in the arc discharge for both the transferred and non transferred mode. In each case three industrial gases are compared, nitrogen, air and oxygen, at different flow rates and currents. T_e is determined from the Balmer line ratio, wherein 5% hydrogen gas is added to the working gas in the torch flow. Variation of the emission with torch height and across the arc radius will be discussed. Recently, free arcs have shown evidence of non-LTE behavior in the arc mantle. Comparison of arc emission spectra as a function of radius for the transferred vs.\ non-transferred modes will be reported. Calorimetry results for the chamber walls, exhaust, and waste crucible will be correlated with the spectral results. This work was supported by the Office of Naval Research.

[J15.08] Study of the \boldmathN_2 + O^+ \leftrightarrow NO^+ + N reaction through the Ã\boldmath^2 \Sigma ^+ excited state of \boldmathN_2O^+.

The reaction N_2 + O^+ \leftrightarrow NO^+ + N, which plays an important role in the upper atmosphere, has been the subject of many studies. In spite of all of these works, this reaction is not completely understood from a quantum mechanics point of view.

Since this reaction is very complicated to study directly, we used a fast beam of N_2O^+ in its ground \~X^2\Pi state. After excitation by laser absorption in the Ã^2\Sigma ^+ excited state, we were able to mesure the kinetic energy released to the O^+ and NO^+ fragments.

The results show that through the à state the diatomic fragments are produced in high vibrational levels, which can not correspond to the upper atmospheric conditions.

This confirmes the assumption that the reaction involves preferentially a quartet excited state of N_2O^+ lying below the à state.

[J15.09] Long Range Studies of the G^1II_g and F^1\sigma ^+_g States of Li_2

This abstract was not submitted electronically.

[J15.10] Rydberg and Autoionization Spectra of High np States of Atomic Carbon.

This abstract was not submitted electronically.

[J15.11] Soft X rays from high intensity laser driven plasmas

High density plasmas are produced by illuminating solids with ultra-short laser pulses of 120 fs at 800 nm light, focused down to an intensity of approximately 8x10^16 W/cm^2. Soft X-Rays are thus generated from the subsequent cooling of the plasma. Spectral data is presented for Aluminum, Pyrex, and quartz targets. Emission from up to the Lithium-like species of all targets were observerd and classified.

[J15.12] The spatial profile and the 147 nm radiative efficiency of Xe Surface Wave Discharges

The spatial emission profile and the radiative efficiency of generating 147 nm Xe resonance radiation in a low-pressure, high-frequency surface wave sustained plasma have been investigated. The radiative VUV power output has been obtained from optical absorption spectroscopic measurements of the Xe resonance level population and from Monte Carlo calculations of the effective decay rate of this level. Measurements of the RF power absorbed by the plasma enable the determination of the absolute efficiency of the Xe surface wave discharge in generating VUV radiation. Results show peak efficiencies as high as 0.80 (11).

[J15.13] The Oxidation States in Organic and Inorganic Sulfur Compounds by Soft X-ray Absorption Spectroscopy

X-ray absorption spectroscopy provides a means to perform non-destructive and direct determination of the sulfur oxidation states in organic and inorganic compounds. In this paper a systematic study of sulfur-K near edge measurements of several sulfates, thiosulfates, and sulfides taken at B.L. 9.3.1 of the Advanced Light Source (ALS) will be presented along with the calculated spectra. These measurements can be used as a standard for estimation of the oxidation state of naturally occurring sulfur compounds.

[J15.14] Multi-ion Coincidence Measurements of Deep-Core-Excited CH_3Cl

Using light from B.L. 9.3.1 at the Advanced Light Source (ALS), the relaxation dynamics of CH_3Cl were studied. A space-focused time-of-flight mass spectrometer was used to take multi-ion coincidence measurements following the excitation in the neighborhood of Cl K-shell threshold (approximately 2.8 keV). Analysis of the data gives insight into the fragmentation mechanisims, and shows preferential fragmentation localized around the initially excited site.

[J15.15] A Unitary Transformation in the Contracted Symplectic Model Approach

In the last years a contracted version of the Symplectic Shell Model scheme has been used to describe the energy spectra and electromagnetic transitions to describe light and heavy rotational nuclei. In these works a model hamiltonian that takes into account the shell structure, couplings to major shells through a quadrupole-quadrupole interaction, and a residual rotor term were used. In the present contribution a unitary transformation is introduced, which gives rise to a simpler hamiltonian and the matrix elements of its different component terms with respect to the U_b \times U_s(3) basis states are easily calculated. Also the quadrupole electromagnetic transitions can be easily determined. At the same time this unitary transformation in the boson approximation limit yields new insights to the shell model interpretation of the quantum rotor hamiltonian.

[J15.16] Excimer Emission from Microhollow Electrode Discharges

Gas discharges between electrodes with a sub-millimeter anode-cathode gap and with a submillimeter opening have been studied, both in argon and xenon. By reducing the gap and the hole diameter in the electrodes, the pressure range of these discharges has been extended up to atmospheric pressure. Below approximately 100 Torr, the discharge behavior is closely described by what is termed a "hollow cathode" discharge (A. Fiala et al., XXII ICPIG, Contr. Papers 4, p. 191, (1996)). Hollow cathode discharges in the subTorr to Torr range are known to contain a high percentage of high energy electrons (K. Fujii, Jpn. J. Appl. Phys. 16, 1081 (1977)). At pressures above 100 Torr the discharge behavior departs from that of a hollow cathode discharge. Highly energetic electrons, however, are still present. This is evidentiated by detection of excimer radiation at 172 nm (xenon) and 128 nm (argon). A three order of magnitude increase has been observed between 100 and 750 Torr, consistent with the excimer formation mechanism, a three-body process involving energetic electrons and gas atoms.

[J15.17] Fluorescence Excitation and Depletion Spectroscopy of the BAr Complex: Electronic States Correlating with an Excited Valence State.

Laser fluorescence excitation and depletion spectra of the BAr van der Waals complex, in the spectral region around the B atomic 2s2 p^2 ^2D \leftarrow 2s^22p ^2 P transition at 208.9 nm, are reported. The fluorescence excitation spectrum displays a series of structured bands, whose excited levels are weakly bound (\leq 60 cm^-1), and an unstructured continuum to the blue. The onset of the latter allowed determination of the ground state dissociation energy D_0'' = 102.4 \pm 0.3 cm^-1. The fluorescence depletion spectrum reveals, in addition, bands at lower wavenumbers to a strongly bound, predissociating state, assigned as C^2\Delta. The bands observed by fluorescence excitation are assigned to the D^2\Pi, E^2\Sigma^+ \leftarrow X^2\Pi transitions. The derived spectroscopic data are used to construct potential energy curves for the C^2\Delta and D^2\Pi states. The predissociation of the C^2\Delta state is due to spin-orbit coupling to a repulsive ^4\Pi state correlating with the B(2s2p^2 ^4 P) + Ar asymptote. A weak nonradiative decay process is also observed for the D^2\Pi state.

[J15.18] Electronic Spectroscopy of AlAr_n Clusters: Evidence for Surface Binding of Al Atoms

Solvent-induced-frequency shifts and splittings of the 3p \rightarrow 3d transition of Al atoms in Al(Ar)_n clusters were studied to investigate the site occupied by the Al atom in these clusters. The electronic spectra were recorded near 300 nm for several sizes in the range n = 1 to 60 by resonant two-photon photoionization spectroscopy. The observed bands differ significantly from UV absorption spectra of Al atoms in Ar matrices, and the variations in splittings and shifts as a function of cluster size do not converge on the matrix limit. These results are interpreted with a model in which the Al atom is bound to the surface of an Ar_n cluster rather than solvated within the cluster interior.

[J15.19] SIMION3D Electron Optics Studies of Zero-Degree Auger Projectile Electron Spectrometers

The electron optics of two different electron analyzers: a tandem parallel-plate electrostatic analyzer and (2) a hemispherical electrostatic analyzer with a four-element deceleration lens and a 2-dimensional position-sensitive detector are studied using an ion optics program.(Simion 3Dv6.0 by D.A. Dahl.) The simulations include studies of energy resolution and transmission for electrons retarded prior to energy analysis, effects of slit fringing-fields, field ionization inside the spectrometers and effective solid-angle calculations under deceleration for the detection of long-lived Auger states. This is a report of work currently in progress and results to date will be presented.

[J15.20] A Zero-Degree Hemispherical Electron Spectrometer with 2-Dimensional Position-Sensitive Detector

An electrostatic hemispherical analyzer with a mean radius of 101 mm using a 2-dimensional position-sensitive detector, resistive-anode encoder and, a four-element deceleration lens has been set up to perform high resolution zero-degree Auger projectile spectroscopy (ZAPS) in ion-atom collisions. This spectrometer is expected to provide much-improved overall electron-detection efficiency compared to conventional ZAPS tandem parallel-plate analyzers. This is a report of work currently in progress and results to date will be presented.

[J15.21] The KSU-CRYEBIS, a User Facility for Low Energy, Highly Charged Ions

The KSU-CRYEBIS, a CRYogenic Electron Beam Ion Source, delivers ions with a wide range of species, charge states, and atomic masses. The present ionization power of 9.9~keV allows for the production of Ar^18+, Kr^34+, and Ta^50+ with time-averaged currents in the pA range. Time-averaged currents up to several nA can be obtained for certain lower charge states of light ions. The CRYEBIS is located on a high voltage platform, which allows one to accelerate the ions up to 160~keV/q or to decelerate the ions to a fraction of a keV/q. The source is operated in a batch mode, producing pulsed ion beams. However, effective duty cycles in excess of 50% can be obtained with a computer controlled "DC batch mode" (see B.~Walch and M.~Stöckli, this conference). Micro-bunching, in combination with chopping, can produce sub-\mus bunches desired for experiments which require time-of-flight analysis. New users will be accommodated.

[J15.22] Creation of a monoenergetic pulsed positron beam

We have developed a versatile, pulsed source of cold (0.018~eV FWHM), low-energy positrons (E \approx 0--9 eV). Ten microsecond duration pulses of 10^5 positrons are extracted from a thermalized, room temperature positron plasma stored in a Penning trap. The frequency, duration, and amplitude of the pulses can be varied over a wide range. This technique allows for generation of electron pulses by simply inverting the polarity of all electrode potentials. Using this technique, we have also generated quasi steady state electron and positron beams with comparable energy resolution. There are numerous potential applications of such a source of intense, cold positrons. Examples include material surface characterization, such as depth profiling, positron and positronium gas scattering, and annihilation studies.

[J15.23] Laser Doppler Velocimetry of a Fast Atomic Lithium Beam

We describe a method for measuring the velocity of a fast atomic lithium beam using a temperature-controlled, planar, calibrated, solid etalon. A 670 nm dye laser is directed antiparallel to the atomic beam and sent through the etalon. While scanning over the 2s~^2S_1/2~-~2p~^2P_3/2 (D_2) transition, a photomultiplier tube detects the Doppler-shifted fluorescence, and a photodiode detects the signal from the etalon. Additionally, a 670 nm diode laser, tuned to the 2s~^2S_1/2~-~2p~^2P_1/2 (D_1) transition transversely excites a thermal lithium beam and the fluorescence spectrum gives the unshifted resonance peaks with respect to the etalon fringes. The velocity of the beam is directly proportional to the unshifted D_2 wavelength(L.\ Windholz, \textitet.\ al.\ )Z.\ Phys.\ D \textbf29, R1976(1994)., the Doppler shift of the fast-beam resonance(P.\ M.\ Koch, Opt.\ Com.\ \textbf20), 115(1977)., and the free spectral range of the etalon. The result gives a precision of 0.03%, an improvement over our previous precision of 0.14%, obtained using a commercial wavemeter to measure the Doppler-shift(R.\ J.\ Rafac, \textitet.\ al.\ )Phys.\ Rev.\ A \textbf50, 121(1994)..

[J15.24] Preparing Francium for Laboratory Experiments

We describe a technique for delivering a beam of francium from a replenishable 10 day half-life source. Starting with a thin layer of ^229Th (T_1/2 = 7300 y), we electrostatically collect the ^225Ra (T_1/2 = 15 d) daughter, which in turn decays to ^225Ac (T_1/2 = 10 d). The Ra/Ac is placed in an orthotropic oven(T. Dinneen, A. Ghiorso, and H. Gould, Rev. Sci. Ins., 67, 752 (1996).), where the decay of the Ac to ^221Fr (T_1/2=4.9 m) is the source of the francium which exits the orthotropic source as a highly collimated thermal beam. Actual and theoretical performance of this system will be compared.

[J15.25] Ray Tracing Demonstrates the Importance of Focusing in the Practical Operation Near Cutoff of an Electrostatic Filter Lens

In our fast-beam apparatus we have long used( P. Koch and K. van Leeuwen, Phys.\ Rep.\ 255), 289 (1995). an electrostatic filter lens (FL) selectively to transmit energy labeled signal ions (e.g., H^+ or He^+) whose energy E_B + E_L is E_L=40--300 eV above the energy, typically E_B=14.6 keV, of the much more intense primary ion beam. Based on one originally used( H. Zeman, K. Jost, and S. Gilad, Rev.\ Sci.\ Inst.\ 42), 485 (1971). with hundred-eV-energy-range electrons, our 12.8 cm long FL has 21 identical, equally spaced, 0.1 cm thick mumetal disks (11.4 cm OD with 1.91 cm dia. axial hole) electrically biased via resistors so that its near-axis electrostatic field is approximately hyperbolic. We have long noted that the analysis presented in Ref.\ [3], which ignores focusing effects, fails to explain why our FL has a final cutoff up to five or more times sharper than `theory'. We present ray tracing results obtained with the computer program \sc Simion to show that strong focusing and higher operating regions (initially parallel rays crossing the axis more than once) play a very important role in the operation of the FL near cutoff and in sharpening its cutoff. Agreement is good.

[J15.26] Reflective Linear Polarization Rotator that Preserves Input-Output Collinearity

We present the design, construction and testing of a device that uses four metallic mirrors to rotate the linear polarization of an optical beam while preserving the input-output collinearity. (Our work corrects design errors in previous work.(L.L. Smith and P.M. Koch, J. Opt. Soc. A, 13), 2102 (1996).) The design uses the concept of geometric phase to find a simple and symmetric arrangement of mirrors that, through a series of oblique reflections, rotates the linear polarization by an angle \phi. We report on the successful constructuction of a \phi=\pi /2 rotator using four uncoated gold mirrors and its testing with a CO_2 laser beam. Its design was independently verified by ray-tracing calculations based on classical electromagnetic-wave theory. We found a small but measurable degree of ellipticity in the output beam. When using metallic mirrors with dielectric overcoatings, such as protected Al mirrors, we measure a nonnegligible degree of ellipticity. Due to the small difference in s- and p-polarization phase shifts, acquired upon reflection from metallic mirrors, our polarization rotator is most suited for the mid- and far-infrared.

[J15.27] Secondary Electron Emission Effects in Selective Field Ionization of Na Rydberg Atoms

A common technique of detecting Rydberg atoms is selective field ionization (SFI). It consists of ionizing Rydberg atoms with an electric field pulse (usually generated between two metal plates). The resulting electrons or ions get accelerated by the field, pass through small aperture(s) in one of the plates, and get detected with a charged particle detector located behind this plate. Different states are then observed as peaks in the detector signal as a function of time. When electrons are detected, the corresponding ions accelerate to the opposite SFI plate and knock out electrons via kinetic or potential emission processes. These secondary electrons then get accelerated toward the detector and manifest themselves as additional peaks in the signal. Depending on signal resolution, these extra peaks may or may not be obvious. This may seriously affect experimental results, especially those which take the state populations into consideration. We report on high resolution studies of this effect with Rydberg s-states of Na by varying n between 19 and 30, (2) the point of excitation of the atoms in between the SFI plates, and (3) SFI plate material. Our results compare well with calculations and other data in kinetic electron emission. We find that the secondary electron signal is important for n\leq 23.

[J15.28] The Aurora Borealis: a plasma physics and environmental laboratory in the sky

The auroral Borealis is Nature's most colorful display of plasma physics = in the polar region. This talk will describe the free energy sources in = terms of particle precipitation and associated current in the polar magnetic = field responsible for such phenomena. A "High Power Auroral Stimulation" = (HIPAS) facility in Alaska has been used to actively probe this auroral region by exciting plasma resonances remotely. This laboratory without walls = allows large-scale controlled perturbations to be created and monitored. The auroral current and particle distribution can be the sources of = excitation or enhancement of plasma waves which are detected by satellites, lidars , holographic arrays, space-borne and ground-based optical and = electromagnetic sensors. The science and application of this active interaction method to environmental monitoring and remediation will be discussed.

^1. A.Y. Wong et al. Phys Rev Lett v63, 271 ( 1989).

Work supported by ONR and NSF.

[J15.29] Development of a Cryogenic Hydrogen Maser and a Double-Bulb Rubidium Maser

We are developing new maser devices with applications in basic and applied physics: a cryogenic hydrogen maser and a double-bulb rubidium maser. The cryogenic hydrogen maser (\textscchm) operates at 0.5 Kelvin, employing superfluid helium coated walls to store the hydrogen atoms. The frequency stability of the \textscchm may be significantly better than a room temperature hydrogen maser because of greatly reduced thermal noise and larger signal power. We are also using the \textscchm to study cold hydrogen-hydrogen and hydrogen-helium collisions. The double-bulb rubidium maser (\textscdbrm) has two connected glass chambers, one for optical pumping of rubidium atoms and one where a maser signal can be produced on the ^87Rb hyperfine transition. The novel design of the \textscdbrm will effectively eliminate the optical pumping light shift and buffer gas pressure shift that limit conventional rubidium frequency standards. The \textscdbrm may provide short term frequency stability superior to that of a room temperature hydrogen maser, but in a smaller and more robust unit. We are currently studying the effects of wall-coatings on the polarization loss and decoherence rates for the optically pumped Rb atoms. Recent experimental results with the \textscchm and \textscdbrm will be reported.

[J15.30] Demonstrateion of a Stable and Tunable Atomic Laser Frequency Standard

We have developed and characterized a dichroic atomic vapor laser lock (DAVLL) using a rubidium vapor cell in a constant magnetic field (100 G). Linear light passes through the cell and is then separated into right- and left-circular polarizations. The power in each circular polarization is monitored with a photodetector, and when the two signals are subtracted, a broad, stable differential signal is generated. A lock to this signal offers many advantages, including a 1 GHz recapture range and tuning range. It's drift rate is .5 MHz/12 Hrs. Anecdotally, we maintained a Magneto-Optical Trap for a week without relocking or tuning the lasers to a DAVLL signal. This lock is relatively inexpensive, for it requires much less insturmentation than other locking schemes with similar tunability.

[J15.31] Large Scale Polarization of Noble Gas for Use in Medical Imaging

The use of spin-polarized noble gases for magnetic resonance imaging and spectroscopy has attracted a great deal of attention recently, especially in the biomedical community. However, experimentation has been limited by the availability of polarized gas. In our lab, we are investigating theoretically and experimentally the factors that determine the quantity of polarized gas (^129Xe, in particular) that can be produced and delivered. Furthermore, we have a parallel effort devoted to the construction of a large quantity polarization apparatus that would deliver polarized gas in quantities suitable for human inhalation studies (> 1 L). Recent progress in these efforts will be reported.

[J15.32] NMR Studies of Porous Media and Complex Fluids Using Laser Polarized Noble Gases

We are investigating the use of laser polarized spin \frac12 gases, ^3He and ^129Xe, as NMR probes of porous media and complex fluid systems. The laser polarization technique increases the NMR signal-to-noise ratio by about 10,000. In contrast to water and other liquids, a gas phase NMR probe has a larger diffusion coefficient and may sample a greater number of pores during the experimental observation time; this should allow improved micro-porosity determination. We are pursuing NMR studies, including imaging and measurements of restricted diffusion and diffusion persistence, for systems such as sandstone rocks, foams, granular media, and the trabecular structure of bone. The high solubility and large chemical shifts of ^129Xe in lipid solutions may make it a powerful liquid phase NMR probe when dissolved in systems such as emulsions, liquid crystalline polymer films, and flowing multi-component fluids. Recent experimental results will be reported.

[J15.33] Relativistic Many-Body Perturbation Calculations of Transition Rates for Atoms with Two Valence Electrons

Relativistic many-body perturbation (MBPT) calculations have been applied to determine energy levels for closed-shell atoms, as well as for atoms with one, two, or three valence electrons.( M. S. Safronova, W. R. Johnson, and U. I. Safronova, Phys.\ Rev.\ A54), 2850 (1996). Relativistic MBPT calculations have also been carried out to study transition rates in atoms with one valence electron.(V. A. Dzuba, V. V. Flambaum, and O. P. Sushkov, Phys.\ Lett.\ A142), 373 (1989). In this paper, we apply relativistic MBPT to investigate transition rates for atoms with two valence electrons. Formulas from relativistic MBPT are given for second-order transition amplitudes in atoms with two valence electrons. Transition rates for Be-like ions are determined to second order in relativistic MBPT.

[J15.34] Fluorescence Decay of the a^4\Pi _u Metastable State of O_2^+.

A cylindrical radio-frequency ion trap(Located at the Harvard-Smithsonian Center for Astrophysics) was prepared to store a population of O_2^+ ions in an effort to observe the fluorescence decay of the a^4\Pi _u metastable state of O_2^+.(This work supported in part by Research Corp. Grant No. CC4323.) A metastable O_2^+ ion population was produced inside the cylindrical rf trap by electron bombardment of O_2 at pressures ranging from 4~to~8\times 10^-8~Torr. After the ions were created, the radiative decay of the population was monitored versus time by focusing a fraction of the light emitted by the decaying metastable ions onto a narrow-band (\Delta \lambda \leq 10~nm) interference filter in front of a photomultiplier tube operated in a photon-counting mode. Two different interference filters were used: peak transmittances were at 235- and 265-nm, respectively. Fluorescence decay curves resulted at all pressures and with either interference filter, as long as the trap was tuned for O_2^+ storage. The decay curves indicate at least two primary decay channels exist for the decaying population. This observation corroborates a preliminary report that monitored the fluorescence decay of an O_2^+(a^4\Pi _u) population confined to a Kingdon trap, and also, one of several published lifetime measurements, which did not employ the observation of fluorescence decay.

[J15.35] Oscillator Strengths for Fine-Structure Transitions in S III

Oscillator strengths and transition probabilities for transitions among the fine-structure levels of the terms belonging to the 3s^23p^2, 3s3p^3, 3s^23p3d, 3s^23p4s, 3s^23p4p, and 3s^23p4d configurations of S III are calculated using configuration-interaction wave functions. The relativistic effects in intermediate coupling are incorporated by means of the Breit-Pauli Hamiltonian. Small adjustments to the diagonal elements of the Hamiltonian matrices have been made so that the energy splittings are as close as possible to the experimental values. The present results for the excitation energies and oscillator strengths will be compared with the available other calculations and the measurements.

[J15.36] Calculation of energy levels and oscillator strengths for the electric-dipole transitions of sulfur.

A large-scale configuration interaction (CI) calculation has been performed for the sulfur atom to obtain excitation energies and oscillator strengths of some electric-dipole transitions. In the calculation relativistic effects were included through the Breit-pauli approximation. A fine-tuning technique was used for the adjustment of the diagonal matrix elements to achieve accurate energy levels and coefficients of the CI wavefunctions. The calculated excitation energies are mostly within 1% of the measurements. The agreement between the length and the velocity forms of the oscillator strengths is also very good.

[J15.37] Accurate MCDF Calculations of Transition Probabilities

Results from large-scale multiconfiguration Dirac-Fock and configuration interaction calculations including the Breit interaction are presented for intercombination and forbidden transitions in C~III and in the Mg iso-electronic sequence. In the calculations the orbital sets of the initial and final state wave functions were not restricted to be the same, but were optimized independently to ensure an accurate description of both states. The evaluation of the transition matrix elements was done using a technique where the two orbital sets are transformed so as to become biorthonormal, in which case standard Racah algebra can be used. The predicted transition probability for the 2s^2~^1S_0 - 2s2p~^3P_1 transition in C~III is A=102.9\pm 1.3 s^-1 and is in excellent agreement with with a recent storage ring measurement. For the 3s^2~^1S_0 - 3s3p~^3P_1 transition in Al~II and Si~III the theoretical values of the transition probabilities are lower that experimental values from ion trap experiments.

[J15.38] Lifetimes of the 3p^54s ^3P_1, ^1P_1 and 3p^53d ^3P_1 levels in K II

Oscillator strengths for the three lowest ground state transitions 3p^6 ^1S_0 - 3p^54s ^3P_1, 3p^6 ^1S_0 - 3p^54s ^1P_1 and 3p^6 ^1S_0 - 3p^53d ^3P_1 in argonlike K II are reported. Theoretical studies show that the specification of the 3p^54s lifetimes in K II and Sc IV involve special problems not encountered for other members of the Ar sequence. The plunging of the 3p^53d levels, caused by the collapse of the 3d orbital, leads to strong configuration interaction for K II where 3p^53d ^3P_1 is interposed between the 3p^54s ^3P_1 and ^1P_1, and for Sc IV where the 3p^53d ^1P_1 lies just above the 3p^54s ^1P_1. The lifetimes were measured using foil excitation of a fast ion beam, which yielded values \tau (4s ^3P_1)=8.4\pm 1.6 ns, \tau (3d ^3P_1)=2.4\pm 0.4 ns, and \tau (4s ^1P_1)=0.43\pm 0.04 ns. Application of these results to the development of XUV lasers is discussed.

[J15.39] Energies and Oscillator Strengths for Lithium-Like Ions

The Hylleraas-type variational method \footnote[1]Z.-C. Yan and G. W. F. Drake, Phys.\ Rev.\ A 52, 3711 (1995). \footnote[2]Z.-C. Yan and G. W. F. Drake, Phys.\ Rev.\ A 52, R4316 (1995). is used to calculate the energies of the lithium 2S and 2P isoelectronic sequences up to Z=20. The oscillator strengths for the 2S-2P transitions are calculated for Z up to 20 and the corresponding lifetimes are determined. The large-Z expansion method is also applied to these calculations and the results are compared to the variational ones.

[J15.40] UV and VUV Oscillator Strengths for Co II

VUV oscillator strengths for Co II are being measured with the high sensitivity absorption experiment at the University of Wisconsin. This experiment uses a hollow cathode discharge as an absorbing sample, the Aladdin storage ring at the Synchrotron Radiation Center as a continuum source, and a 3m focal length vacuum echelle spectrometer equipped with a CCD detector array. The experiment achieves spectral resolving powers of 350,000 and sensitivities to fractional absorptions much smaller than 1% at deep UV and VUV wavelengths. Column densities of sputtered metal atoms and ions as small as 3x10^8 cm^-2 can be detected. The absorption experiment measures relative oscillator strengths for lines from a common lower level, usually a VUV transition relative to a well known UV transition. The reference UV oscillator strengths are determined by combining emission branching fraction measurements made on the 3m echelle spectrograph with accurate radiative lifetime measurements made using laser induced fluorescence. This experiment is applicable to essentially every element in the periodic table, both neutral atoms and atomic ions.

[J15.41] Postfoil Ion Beam Divergence Calculations Utilizing the Tabulations of Sigmund and Winterbon

Calculations provided by Sigmund and Winterbon's implementation of Bothe's theory concerning small angle and multiple scattering can be utilized for postfoil divergence estimates in Beam-Foil Spectroscopy. An ANSI C program, SIGMUND, has been written to estimate this divergence by using the scattering tables of Sigmund and Winterbon. This program uses a superset of PC specific utilities and graphics libraries of the Borland Graphics Interface. SIGMUND reads as input an ASCII version of the Sigmund and Winterbon data tables, loads this into a suitable data structure, and provides a facility to convert the reduced units of physical units. Options include printing the full scattering distribution data in tabular form, graphing the data on a PC screen and also interpolating the data for a specific foil thickness with subsequent tabulation and graphing. An additional option calculates the relative concentration of randomly distributed defects for a crystalline target, based on the dechanneling model of Bogh.

[J15.42] High Resolution VUV Laser Measurements of the Band Oscillator Strengths of the CO A^1\Pi (9\leqv'\leq17) - X^1\Sigma^+ (v"=0) Transition.

Band integrated oscillator strengths of the CO A^1\Pi - X^1\Sigma^+ (v'-0) transition for 9\leqv'\leq17 have been measured using a high resolution VUV laser as background source. The rotationally resolved absorption spectra are least squares fitted by precise synthetic spectra in order to take into account the instrumental saturation effects. The fitting parameters are the oscillator strength, the rotational line width and the instrumental width. Saturation effects are reduced by pressure broadening the absorption lines using up to 1.5 bar of Argon as a buffer gas. The new data set extends the range covered by recent measurements and is in general agreement with them and with ab initio calculations. The average broadening coefficient of the CO rotational lines by Argon is determined in this study for the first time : 2\gamma_CO-Ar=1.37 10^-7 s^-1 cm^3.

[J15.43] Absorption Measurement of the Ratio of Cesium 6^2P_J Oscillator Strengths

We report a precise measurement of the ratio of the squared modulus of the matrix elements for the 6^2S_1/2~-~6^2P_1/2,3/2 transitions in cesium via absorption of near-resonant laser radiation. Light from a single-frequency scanning Ti:S laser traverses an absorption cell which is temperature controlled and stabilized to <~5~mK\@. Power normalized absorption spectra are recorded for detunings of \approx~3~GHz near each fine structure component in successive passes. Precise laser frequency information is obtained in parallel by observing the fluorescence of a cesium thermal beam probed by the same laser light with three additional orders of sidebands, which are generated in a 1~GHz electrooptic phase modulator. The spectra are analyzed using a comprehensive model of the line profile including impact-regime self broadening effects. The results provide a stringent test of high-accuracy atomic structure calculations and are complementary to our program of fast-beam alkali lifetime measurements(R.~J.~Rafac \itshape et al.)\ Phys.\ Rev.\ Lett.\ \bfseries 50 R1976 (1994); accurate knowledge of the structure of low-lying levels in atomic cesium is of great importance to the interpretation of recent measurements of parity non-conservation in this system.

[J15.44] Two-Photon Decay of 2\, ^1\! S_0 and 2\, ^3\! S_1 States of Heliumlike Ions.

We present the results of relativistic CI calculations of two-photon decay of 2\, ^1\! S_0 and 2\, ^3\! S_1 states in heliumlike ions. The calculations include determination of the transition rates and photon energy distributions. This work extends the nonrelativistic calculations of Drake(G.W.F Drake, Phys.\ Rev.\ A34), 2871 (1986) and of Drake, Victor and Dalgarno( G.W.F Drake, G.A. Victor, and A. Dalgarno, Phys.\ Rev.\ 186), 25 (1969). to include the retardation effects and relativistic CI wave functions with the Breit interaction. Calculations are made for the ionic charges Z in the range 2 to 100 and the interpolation formulas are given. The change in shape of the photon energy distribution function with increasing Z is described. We emphasize the high sensitivity of two-photon decay rates for the 2\, ^3\! S_1 state on the physical details of calculations.

[J15.45] Laboratory Observation of Spin-Forbidden Transitions in P^+: 3s3p^3\ ^5S_2\ -\ 3s^23p^2\ ^3P_2,1

We report the observation of a pair of spin-forbidden resonance lines in singly-ionized phosphorus. These lines at \lambda _air = 2210Å\ and 2196Åare observed in emission from a low-pressure inductively-coupled helium plasma containing a small amount of evaporated P_4. These and other spin-forbidden lines in low-Z ions are of interest not only because of the suitability of such lines for astrophysical diagnostic purposes, but also because these lines have presented a substantial computational challenge to atomic theorists. Accurate experimental determination of oscillator strengths and lifetimes therefore provide an excellent test of the various computational approaches. Unfortunately, experimental challenges, such as the inherent weakness of spin-forbidden lines and the difficulty in making sources for several of the species of interest, have prevented many of the desired measurements from being made. We report on our progress toward the measurement of an accurate branching ratio for the 3s3p^3\ ^5S_2\ -\ 3s^23p^2\ ^3P_2,1 spin-forbidden pair in P^+. The branching ratio, when obtained, can be combined with the previously measured ^5S_2 lifetime(A.G. Calamai et al., Phys. Rev. A, 45), 2716 (1992). to yield absolute oscillator strengths for both lines.

[J15.46] Measurement of magnetic-dipole transition probabilities using an electron beam ion trap

The transition probabilities for magnetic--dipole transitions in highly charged ions are measured using an EBIT. In particular the 3d^4\ ^5D_2\leftarrow ^5D_3 transition probability in Ti--like Xe (Xe^32+) has been measured. Our result for this transition corresponds to a lifetime of 2.15(14) ms is in good agreement with the theoretical value of 2.4 ms. Transition probabilities corresponding to Kr XXIII and Ar XIV are also presented.

[J15.47] Magnetic-field effects on the ^1P^\circ shape resonance of H above the H(N=2) threshold.

This abstract was not submitted electronically.

[J15.48] Applicability of the Complex Scaling Method to Calculations of Stark Resonances

Our numerical calculations of the Stark effect in hydrogen suggest a limitation on the range of energies of Stark resonances that can be calculated by the Complex Scaling Method (CSM). We find that CSM can only be used to calculate resonances with energies Im\,E > \sqrt3\,Re\,E. This limitation corresponds precisely to the range of validity of the Herbst-Simon(I. W. Herbst and B. Simon, Phys. Rev. Lett. 41), 67 (1978). theory of the Stark effect in hydrogen. Regions of the complex energy plane beyond this area are not covered by the theory. We have found that we are unable to obtain stablization of variationally-calculated eigenvalues with CSM beyond this range. Such resonances do exist, even for very small electric fields, where they correspond to the more highly-excited energy levels. For stronger fields, however, even the lowest states of atoms will be shifted in energy beyond this limit, and may thus be inaccessible to CSM. Examples of these calculations can be seen at http://www.cs.siu.edu/\~~todorg/stark.html/.

[J15.49] Diamagnetic Hydrogen Energy Level Reordering as a Function of Angular Momentum

Dimensional perturbation theory (DPT)(Dimensional Scaling in Chemical Physics), edited by D.R.\ Herschbach, J.\ Avery and O.\ Goscinski (Kluwer Academic, Dordrecht, 1992). is a powerful calculational and descriptive tool for studying atomic and molecular systems. For the diamagnetic hydrogen problem DPT is equivalent to large |m| perturbation theory once the trivial paramagnetic term is dropped, since the dimensionality D and the magnetic quantum number, m, only enter the problem through the term \kappa\! =\! D\!+\!2|m|\!-\!1. Here, the Schrödinger equation is expanded about the |m|\!\rightarrow\!\infty limit using the perturbation parameter \delta\!\equiv\! 1/\kappa. Avoided crossings are encountered as |m| changes from the completely separable |m|\!\rightarrow\!\infty limit to finite values, and provide a mechanism for energy level reordering with changes in |m|. To understand these avoided crossings, the dependence of the branch point structure in the complex \delta-plane on the field strength B is examined. A parameterization is formulated that smoothly describes the trajectory of these branch points in the complex \delta-plane as a function of B, including non-analytic behavior at Fermi resonances.

[J15.50] On the Origin of the Exact Interdimensional Degeneracies

A characteristic feature of quantum mechanics in D dimensions is the presence of exact interdimensional degeneracies. For one-electron systems states related by the dimensional link D, l \leftrightarrow (D-2), (l+1) are exactly degenerate(D.R.\ Herrick, J.\ Math.\ Phys).\ 16, 281 (1975).\addtocountermpfootnote1. For the two electron problem Herrick and Stillinger found exact interdimensional degeneracies between the D=3 ^1,3P^e and ^1,3D^o states and the D=5 ^3,1S^e and ^3,1P^o states respectively^2,3\footnotetextD.R.\ Herrick and F.H.\ Stillinger, Phys.\ Rev.\ A 11, 42 (1975).. More recently Dunn and Watson have developed a general theory for higher angular momentum states of N-particle systems in D dimensions(M.\ Dunn and D.K.\ Watson, Ann.\ Phys.\ (NY)) 251, 266 (1996); 319 (1996). and derived the complete spectrum of exact interdimensional degeneracies of the two electron atom(M.\ Dunn and D.K.\ Watson, Few-Body Sytems), in print.. Despite this, the underlying reasons for the exact interdimensional degeneracies have remained obscure. We address this issue and relate the exact interdimensional degeneracies to dynamical groups of the one- and two-electron problems.

[J15.51] Precision fast-beam/laser experiments

We have developed an improved system for measuring precise hyperfine structures and decay curves in neutral and low-charged (1+,2+ ) atomic ions using CW laser-excitation of 30 to 200 keV ion beams. A new light-collector with enhanced solid angle collection efficiency is in the testing stage for lifetime measurements. These measurements utilize crossed beam laser excitation in neutral and ionized alkalis and in neutral argon. A tunable, single-frequency Ti-sapphire laser enable us to reach the near infra-red transitions of the 4s-4p states of argon (following rubidium vapor excitation to the metastable states). We hope to present data on the argon results at the meeting.

The hyperfine structure measurements incorporate a new elliptical light collection system, with detector at one focus, and the excited ion beam around the second focus. Initial results may be presented at the meeting for singly-charged rare-earth systems.

[J15.52] Preliminary experiments in atomic search for a nuclear anapole moment.

Previous measurements searching for PNC violations in the close-lying opposite parity 2s-2p states of hydrogen have been unsuccessful. A theoretical analysis and summary of these works has been made by Dunford and Lewis ( R.W.Dunford and R.R. Lewis, Phys. Rev. A23, 10 (1981)) who indicate the strong Z-dependence of the effects. An essential part of these calculation is the mixing of the PNC amplitude with weak M1 decay of the 2s state. In hydrogen, this decay is negligible ( the 2-photon decay dominates), but can becomes significant in higher-Z systems ( e. g. Argon 17+). Unfortunately, intense beams of highly-charged argon make such a measurement difficult. We propose to make similar measurement in a high-Z neutral or singly-charged ion, where much greater atom densities are available.

We have found several possible examples of close-lying opposite parity states in xenon and the rare earths which might be appropriate . First we need to measure the relevant M1 and E1 transition rates, plus determine the hyperfine structures, which will yield the needed energy-splittings of the close-lying states, and the mixing parameters. We shall report on the planned fluorescence asymmetry measurement using fast beam/laser excitation.

[J15.53] Recent Results from the Harvard-Smithsonian Dual Noble Gas Maser

The dual noble gas maser (\textscdngm) is the first device to simultaneously sustain active maser oscillations on two distinct transitions in different atomic species. The \textscdngm consists of dense, co-habitating ensembles of spin-polarized ^3He and ^129Xe atoms each performing active maser oscillations on their nuclear spin-\frac12 Zeeman transitions at audio frequencies. The maser population inversions are created by spin exchange collisions between the noble gas atoms and optically-pumped rubidium vapor. The \textscdngm allows sensitive differential measurements of the noble gas Zeeman splittings. Thus one noble gas species can serve as a precision magnetometer to stabilize the system's static magnetic field, while the other species is used as a sensitive probe for new physics such as a permanent electric dipole moment of the ^129Xe atom as a test of time reversal symmetry in elementary particle interactions. A \textscdngm has recently begun operation in our laboratory and has attained ^129Xe maser frequency stability of \sim 100~nHz, and ^3He maser frequency stability of \sim 250~nHz, for a 4000~s averaging time. These results will be presented, along with a description of ongoing efforts to improve the masers' frequency stability.

[J15.54] Two-Photon Doppler-Free Spectroscopy at High Magnetic Field

We have implemented a CW detection scheme to probe ground-state populations of ^85Rb and ^87Rb at magnetic fields in the kilogauss regime. The method is based on two-photon, Doppler-free excitation to the 5d state, which has been used by other groups for a variety of applications, including zero-field spectroscopy. We use an ECDL tuned to 778nm to injection-lock a second, free-running diode laser that provides ample power with narrow bandwidth. The system has been used to test the efficacy of an intracavity AOM-broadened ECDL, which we use to perform optical pumping on rubidium in the intermediate and Paschen-Back regimes. We further study polarization lifetimes at various pressures of several buffer gases in order to maximize spatial confinement of the rubidium while retaining the ability to resolve the pressure-broadened lines. Relevance of the results to a test of time-reversal invariance in a three-state clock will be discussed.

[J15.55] Operation of a Bimodal Waveguide Resonator

We have built a doubly-resonant waveguide cavity to simultaneously excite two magnetic dipole transitions between Zeeman states in rubidium. The resonator has been designed for operation with a rubidium-loaded quartz cell, and such that the two cavity resonances can be independently tuned near 5.3GHz and 6.0GHz without significant perturbation to the magnetic field distribution at the center of the structure. This is important since the resonator will be used to drive transitions in a three-state clock that requires an interaction volume of three low-gradient, RF magnetic fields polarized in a mutually orthogonal configuration. Results from a simulation of the cavity will be presented. Progress toward completing the apparatus (including adding the third magnetic field) will be reviewed.

[J15.56] Work Towards a New Precision Source of Alpha Using Helium Fine Structure

Recent work in atomic theory\footnote T. Zhang, Z-C Yan, and G. Drake, Phys. Re v. Lett. 77, 1715 (1996). and experiment\footnote D. Shiner, R. Dixson, P. Zhao, Phys. Rev. Lett. 72, 1802 (1994).,\footnote D. Shiner and R. Dixson, IEEE Trans. Instrum. Meas. 44 518 (1995). suggests the helium atom could provide a new high precision source of the fine structure constant, alpha. We present an experiment which is underway using a stable external cavity diode laser and a high speed integrated electro-optic modulator. The modulated laser beam drives the triplet 2S-2P transition in a beam of metastable helium atoms with a goal of attaining a precision of 5 ppb in alpha. This precision would, among other things, allow theory and experiment in the electron g-factor to be more fully tested.\footnote T. Kinoshita, Rep. Prog. Phys., to be published. The current status of the experimental will be given.

[J15.57] E1-M1 Damping Interference in the Electric Field Quenching of Metastable Ar^17+ Ions

The metastable 2s ^2S_1/2 state in one-electron ions decays to the ground state either by two-photon or M1 decay. If an electric field E is applied to the ion there is also a ``Stark induced'' E1 amplitude. Interference between the E1 and M1 amplitudes leads to an asymmetry in the angular distribution proportional to the invariant k \cdot E which is of interest because it appears to violate time reversal (T) invariance. (The electric field is even, while the photon momentum k is odd under T.) It has been shown that this term is nonvanishing in a T-conserving theory if damping is taken into account. We have observed this ``damping-interference'' term in an experiment at the Argonne Linac (ATLAS). In the experiment, 370 MeV metastable Ar^17+ ions were quenched in an electric field produced by motion perpendicular to a static magnetic field. The asymmetry of the decay radiation was measured with two Si(Li) x-ray detectors. We will discuss our latest results and their interpretation as a precision measurement of the structure of H-like argon.

[J15.58] Thermalization of Positronium in Gases

The thermalization rate of orthopositronium (o-Ps) in a gas is a major systematic concern in some precision annihilation decay rate measurements(C.I. Westbrook, D.W. Gidley, R.S. Conti, and A. Rich, Phys. Rev. A40), 5489 (1989);Phys. Rev. Lett. \bf58, 1328 (1987). since incompletely thermalized o-Ps could result in an age dependent collisional quenching rate. To investigate this issue, the initial energy and thermalization rate of o-Ps has been directly measured in the same apparatus and gases of Ref. 2 by adding a Ge detector. Time resolved doppler broadening of 2\gamma annihilation radiation from magnetically quenched o-Ps was used to deduce its average energy as a function of age over a range of gas densities. A quasi-elastic thermalization rate and an average initial energy are then deduced. Results will be presented for Ne, Ar, N_2, isobutane and neopentane.

[J15.59] Impact of Orthopositronium Thermalization on Decay Rate Measurements

We quantify the effect of incompletely thermalized orthopositronium (o-Ps) (see previous abstract) on its measured annihilation decay rate.(C.I. Westbrook, D.W. Gidley, R.S. Conti, and A. Rich, Phys. Rev. A40), 5489 (1989);Phys. Rev. Lett. \bf58, 1328 (1987). A determination of the energy (temperature) dependence of the o-Ps collisional quenching rate has been made in each of the gases of Ref. 2 by forming positronium in a high pressure gas cell which could be heated from 20-300^oC. An increase in the collisional quenching rate with temperature was observed for isobutane, neopentane, nitrogen and argon. Significantly smaller changes were observed for neon and helium. These results, together with the independently measured thermalization rates, have been used to determine the systematic effect of incompletely thermalized o-Ps in the previous decay rate measurement.^2 A correction to the result of Ref. 2 will be discussed.

[J15.60] Adiabatic Raman transitions in YbF

We have performed adiabatic Raman transfer(U. Gaubatz et al., Chem. Phys. Lett. 149) 463 (1988) between hyperfine levels of the N=0 rotational state of YbF. We will discuss several advantages of this technique as compared with traditional molecular beam radiofrequency spectroscopy. These Raman transitions can be used as a splitter and recombiner in a spin interferometer which will form the heart of an experiment to measure a permanent electric dipole moment of the electron using YbF.

[J15.61] Resonant Accumulation of Positrons for a Spectroscopy Experiment in Positronium

Chaotic transport to higher transverse energies (E_\perp) has been previously observed in cylindrical Penning traps. The growth in E_\perp stems from non-adiabaticity of axial motion in the presence of endcap radial electric fields and is consequently present, to varying degrees, in all such traps. This transport has been implemented, as a very effective trapping mechanism, in a positron trap intended for a precision spectroscopy experiment in Positronium. Until now optimum accumulation relied on the presence of large radial fields (\sim100~V/cm) near one endcap. Generation of such fields and subsequent extraction of the trapped positrons from the trap is, however, not trivial and produces several experimental difficulties for the spectroscopy measurement. Introduction of smaller radial electric fields (\sim10~V/cm) at a few distinct points in the trap allows far more control over the transport and erases the necessity for large fields. Constructive interference of these smaller increases in E_\perp has been shown to be as effective in trapping positrons as large electric fields. Moreover, this transport can be greatly suppressed, through destructive interference, allowing the associated losses in other traps to be minimized.

[J15.62] Density-Functional Theory with Optimized Effective Potential and Self-Interaction Correction for Autoionizing Resonances.

This abstract was not submitted electronically.

[J15.63] Relativistic All-Order calculations for negative ions.

We present a study for the electron affinities of negative ions where an electron is attached on a closed subshell atom. Our approach is based on multireference relativistic many-body perturbation theory. In particular, we investigate the Ca^-, Sr^-, Ba^- and Yb^- negative ions. The systematic inclusion of correlation corrections beyond second order led us to very good agreement with most recent experiments and a clear improvement over the second order Dyson equation. Comparisons with other many-body calculations are also presented.

[J15.64] A New Way to Calculate the Lamb Shift in Two-electron Systems.

In calculating the Lamb Shift, the Bethe log term, log(k), has always been difficult to evaluate. We use prediagonalized pseudostates(A. K. Bhatia and R. J. Drachman, J. Phys. B 27, 1299 (1994).) as approximations to the exact intermediate states. The sum in the numerator converges very slowly with the number of intermediate pseudostates due to the log term while the denominator converges more rapidly. Also, the denominator can be made independent of the pseudostate sum by using commutation relations and can therefore be calculated quite accurately. This allows us to optimize the nonlinear parameters in the intermediate pseudostates to equal the exact value of the denominator. The essence of the present method is to use the same optimum values of the nonlinear parameters in evaluating the numerator. Results for H^-, He, Li^+, and Be^+2 will be presented and compared with other calculations.

[J15.65] Determination of Nuclear Quadrupole Moments from High-Precision Atomic Calculations

The currently best Q-values(CRC Handbook of Chem. and Phys., 74), p. 9-156 (1993) for many elements are obtained by combining spectroscopic eqQ/h for atoms and molecules with accurate calculations(P. Pyykkö, Z. Naturforsch, 47a), 189 (1992) of q. Our most precise example(M. Tokman, D. Sundholm, P. Pyykkö, and J. Olsen, Chem. Phys. Letters (in press)) is the Q(^14N) of 20.44 mb, obtained using N^2+ (2p; ^2P_3/2) and MCHF calculations. Another case is the Q(^23Na) of 104 mb from NaF and NaCl molecules(P. Pyykkö, A.J. Sadlej, Chem. Phys. Letters, 227), 221 (1994) E-mail: Pekka.Pyykko@helsinki.fi. Further examples will be discussed.

[J15.66] Sturmian functions for two Coulomb centers plus oscillator potential.

Numerical techniques were developed to calculate the Sturmian functions S_n(ømega; q) defined by the equation \beginequation \left\ -\frac12\nabla^2_q+ \rho_n(ømega)V(q)-ømega \right\S_n(ømega; q)=0, \endequation where ømega is a parameter. Sturmian functions are crucial for a consistent formulation of dynamic theories of many atomic processes, such as ion-atom collisions(S.Yu. Ovchinnikov and J.H. Macek, Phys. Rev. Lett. 75), 2474 (1995), Wannier theory(J.H. Macek and S.Yu. Ovchinnikov, Phys. Rev. Lett. 74), 4631 (1995), photoionization(R. Shakeshaft, J. Phys. B 18), L611 (1985); Phys. Rev A 34, 244 (1986). We calculate Sturmians for the potential V(q)= -\fracZ_1|q-\hatk/2| -\fracZ_2|q+\hatk/2| +\fracØmega^22q^2, which appears in the Solov'ev--Vinitsky formulation of collision theory(E.A. Solov'ev and S.I. Vinitsky, J. Phys. B 18) L557 (1985). The solution is facilitated by separation of variables in prolate ellipsoidal coordinates. The calculated Sturmian functions are important components of a complete semiclassical theory of ion-atom collisions. Such a theory includes all dynamic effects, and it has been already constructed for the important particular case of zero impact parameter (no oscillator). The Sturmians calculated in this work allow us to extend the theory to the general case of arbitrary impact parameters.

[J15.67] Computation of Matrix Elements by Uncoupling of Multiconfiguration State Functions

A general N-electron atomic state function | \Psi> can be written as an expansion in terms of uncoupled basis states, each of which corresponds to a product of N fermion creation operators, specified by single-particle quantum numbers \alpha = \n, l, m_l, m_s\, arranged in a standard order: | \Psi>~=~\Sigma~C_\alpha_1...\alpha_N~ a^(\alpha_1)...a^\dagger(\alpha_N)| 0>. We report on a computer program which will form such an expansion for a general configuration state function given initially in the usual LS-coupled notation. The coefficients C_\alpha_1...\alpha_N are computed as square roots of rational fractions, optionally in power-of-prime notation, using only integer arithmetic. Matrix elements of one- and two-electron operators between multi-configuration states can now be evaluated using this expansion as an alternative to traditional methods using Racah algebra. A potential advantage is that the details of the operator in question are separated from the complications of coupling schemes, antisymmetrization and parentage. The program has been checked by computing matrix elements of the electrostatic interaction, and then used to compute spin-dependent two-electron correlation functions. Planned future applications include studies of the Breit interaction, and radiative transitions between states described by nonorthogonal orbitals.

[J15.68] Hyperspherical approach to three-electron atomic systems

A new method for calculating the hyperspherical adiabatic potential energy and its associated channel function of three-electron atoms has been developed. In this method, we first calculate channel functions at each hyperradius for a given set of partial waves of the three electrons using unsymmetrized basis functions constructed from the Discrete Variable Discretization (DVR) method. The couplings of the different sets of partial waves are then taken into account for a selected set of channels to obtain the adiabatic potential curves and the channel functions appropriate to the three-electron atom. The present method eases the burden of numerical treatment for higher angular momentum states. Some results will be presented and discussed.

[J15.69] Allowed and Forbidden Transitions in Singly Ionized Vanadium.

Rates for electric dipole transitions in V II from all levels of the 4p \,z \,^5G^o, z \,^3D^o, z \,^5F^o, z \,^5D^o, z \,^3G^o and z \,^3F^o terms to the levels of the 4s \,a \,^5F and a \,^3F terms have been calculated in a configuration interaction model. All the lower (even parity) levels and certain upper (odd) levels are essentially unmixed by spin-orbit and other relativistic effects while other odd levels are very strongly mixed. Good agreement is obtained with recent observations in the case of the relatively unmixed levels while for some of the strongly mixed levels, the theoretical results are rather poor. We look at numerical aspects of the model involving analysis of the effects of small errors and thereby obtain some insight into the reasons for the difficulty of modelling these transitions.

[J15.70] Electron Affinities of Tb by Attachment of a 6p Electron

This work continues our Relativistic Configuration Interaction study of rare earth electron affinities, U (K. Dinov et~al, Phys. Rev. A 52), 2632 (1995) having been the most complicated to date. A few species [ La (S. H. Vosko et~al, Phys. Rev. A 43), 6389 (1991), Ce (K. Dinov et~al, Phys. Rev. A 50), 1144 (1994), Th (D. Datta et~al, Phys. Rev. A 50), 1107 (1994), Lu (S. H. Vosko et~al, Bull. Am. Phys. Soc. 37) 1089 (1992)] seem to support bound negative ion states of opposite parity by 6p/7p and 5d/6d attachments. The Tb I ground state (4f^9 6s^2) and low lying excited state (W. C. Martin et~al, Atomic Energy Levels - The Rare Earth Elements, Natl. Bur. Stand. Ref. Data Ser, Circ. No. 60), US GPO Washington, DC, 1978 (4f^8 5d 6s^2 at 286 cm^-1) provide a candidate for formation of opposite parity bound states by a common 6p attachment. So far, Multi-Configurational Dirac Fock calculations seem to support this conclusion. RCI calculations for valence correlation will also be reported.

[J15.71] Improved Theoretical Energy Differences between 3d^4 ``^5D''_2 and ``^5D''_3 Levels for Xe^32+, Ba^34+, Nd^38+ and Gd^42+~

The 3d^4 ``^5D''_2-``^5D''_3 M1 transition is of recent interest ^2,3 as a potential plasma diagnostic line due to the near constancy of the wavelength as a function of Z. Experiment (F. G. Serpa et~al, Phys. Rev. A 53), 2220 (1996) and Multi-Configuration Dirac Fock calculations (P. Indelicato, Phys. Scr. T 65), 57 (1996) differ by about 5% (\sim1350 cm^-1), but the latter only includes a limited amount of correlation.

In this work, we report more extensive correlation calculations, using Relativisitic Configuration Interaction methodology, which we have previously applied to the Nb^+ member of this sequence (L. Young et~al, Phys. Rev. A 51), 3534 (1995). Currently, our average error is \sim2.3% (\sim630 cm^-1).

[J15.72] Calculation of the fourth order self-energy contribution to the Lamb shift of hydrogenlike ions

The fourth order pure self-energy contribution to the Lamb shift comes from three Feynman graphs : the crossed rainbow, the double rainbow and the SE-SE graphs. The first two graphs, along with the derivative term from the last, constitute a gauge invariant set. A technique for renormalization of this set will be presented and progress in their numerical evalulation described.

[J15.73] Accurate many electron atomic and molecular calculations made easy

A method of significance to correlated calculations involving several electrons is introduced. All the radial matrix elements in Modified Configuration Interaction (MCI) or correlated calculations ar e formulated as products of simple, independent one-dimensional integrals: one integration between 0 and \infty and all the others from 0 to 1 (there are no longer coupled integrations involving r _ij). The method is easily applicable to any number of electrons in MCI and opens the way to perform direct correlated calculations (no expansions) for more than two-electrons. Very high levels of numerical accuracy for the radial limit of the ground state of helium are obtained using a simple orthonormal basis set: a full 14 digits in double precision and 20 digits in quadruple precision, fa r exceeding in both cases the best previous results. The application of the method to atomic systems with several electrons will be discussed.

[J15.74] Influence of the electron-electron magnetic interaction, of the negative energy continuum and of the vacuum polarization on the hyperfine structure of lithium-like ions ground-state.

With the use of heavy ions storage rings or EBIT, the study of the hyperfine structure of the ground state of ions has lately made enormous progress. Following recent work on the 1s2s\, ^3S_1-1s^2\, ^1S_0 relativistic M1 transitions in helium-like systems, we investigate the effect of the electron-electron magnetic interaction, of the negative energy continuum and of the vacuum polarization on the hyperfine structure of lithium-like ions ground-state. We use a Multiconfiguration Dirac-Fock (MCDF) wave function with all configuration state functions (CSF) originating from all possible combinations of orbitals of principal quantum number 1 to 3. We compare the hyperfine energy of the lithium-like ion for different kind of wave functions. The magnetic part of the Gaunt interaction, or the vacuum polarization can be included in the self-consistent field process. The negative energy continuum can be included or projected out. We show that the contribution of all these terms are of the same order of magnitude, and must be included together with correlation and radiative corrections to provide hyperfine energies with accuracy compatible with laser experiments on storage rings.

[J15.75] Triply Excited States of Lithium Isoelectronic Sequence.

The excitation energies and the probabilities for radiative and radiationless decay of hollow atom states belonging to the 2s^22p, 2s2p^2 and 2p^3 configurations have been calculated on the basis of perturbation theory method (1/Z- expansion) and MCHF approximation (Cowan code) in a broad range of values of atomic number Z. Our results agree well with the first observation of these states(R.Bruch G.Paul and J.Andrä, Phys.Rev.A \bf12),1808 (1975). and theoretical and experimental results obtained recently for neutral Li atom (see, for example, (Y.Azuma et al., Phys.Rev.Lett.,\bf74), 3768 (1995). ^, (S.Cubaynes et al.,Phys.Rev.Lett.,\bf77),2194 (1996).). It should be noted that the 1/Z expansion method gives more accurate results for ions than for neutral system, because we have included only the lowest order coefficients of 1/Z expansion. In this work a good agreement has been achieved for neutral atom allowing us to believe that our data for the same triply excited states with Z>3 should be used for prediction of new experiments with these states.

[J15.76] Multiconfiguration Dirac-Fock calculation of 2s_1/2-2p_3/2 transition energies, branching ratios and cross sections in higly ionized bismuth, thorium, and uranium.

Energies, branching rations and cross sections of 2s_1/2-2p_3/2 radiative transitions in U^89+ trough neonlike U^82+, lithiumlike Th^87+ trough neonlike Th^80+ and lithiumlike Bi^80+ trough neonlike Bi^73+, are calculated including QED effects and correlation. The energy results of the later two sets are compared with recent measurements and with other theoretical calculations, using MCDF, configuration interaction (CI), and Many-Body Perturbation Theory (MBPT). Good agreement with the experiment is found for the cases for which experimental data are available. For each ion, forty-one possible transitions, twenty eight of them for the first time, are identified in the 8 ionization states. We confirmed that the thirteen experimental observed transitions are the ones with the largest relative intensities.

[J15.77] Calculations Of Diatomic Molecules With STO Basis Sets

During the last decade, much progress is made in using Slater Type Orbitals(STOs) as atomic basis set in the quantum treatment of large systems. Indeed, according to early work on the Schrodinger equation, it was shown that STOs yield the best convergence of the variational wavefunctions, since they possess the proper behavior required by the solution of Schrodinger equation, namely, a cusp at the origin (T. Kato Commun. Pure and Appl. Math 10, 151 (1957)) and an exponential decrease at infinity. (S. Agmon, Lectures On Exponential Decay of Solutions of Second Order Elliptic Equations: Bound on Eigenfunctions of N-Body Schrodinger Operators, Princeton University, Princeton, NJ, 1982) However, due to the difficulties of setting up efficient algorithms for multicenter integrals, STOs were considered as sophisticated mathematical objects but of little practical interest. In this work, a general method allowing the calculation of multicenter integrals over STOs is presented. The method which is based on on an addition theorem for STOs and some of its mathematical and numerical properties will be presented. The numerical computations on diatomic molecules, H_2 and N_2 are carried out and compared with results obtained using Gaussian Type Orbitals (GTOs). Our numerical results for such small diatomics are of limited interest, but they are imporatant in fine tunning the computational machinery by comparing them with the data already available for such molecular systems. Furthermore, the conclusions drawn throughout this work will be used as guidlines in developing more general algorithms to treat complicated systems more efficiently.

[J15.78] Long-Range Adiabatic Corrections to the Ground Molecular State of Alkali-Metal Dimers.

The structure of the long-range limit of the diagonal adiabatic corrections to the ground molecular state of diatomic molecules, may be expressed as a series of inverse powers of internuclear distance, R. The coefficients of this expansion are proportional to the inverse of the nuclear mass. Thus, they may be interpreted as a nuclear mass-dependent corrections to the dispersion coefficients. Using perturbation theory we have calculated the long-range coefficients of the diagonal adiabatic corrections up to the order of R^-10. The final expressions are in terms of integrals over imaginary frequencies of products of atomic matrix elements involving Green's functions of complex energy. Thus, in our approach the molecular problem is reduced to an atomic one. Numerical evaluations have been done for all alkali-metal dimers. We acknowledge the support of the U.S. Dept. of Energy.

[J15.79] Modified Configuration Interaction calculations of \mboxH_2^+ and \mboxH_2

A variational approach to molecular calculations is introduced that easily yields accurate non-perturbative values for the electronic-vibrational spectrum of diatomic molecules, avoiding multi-center ed or correlated integrations. With the origin of coordinates at the nuclear center of mass, multi-centered calculations are avoided by expanding the nuclear potential and the electron-electron poten tial as \sum_kr_<^k / r_>^k+1 P_k. If R_i is the radial coordinate of the nuclei and r_i of the electrons then r_< (r_>) is the smallest (largest) of r_i and either R_j or r_j. Fa st variational convergence is obtained using the Modified Configuration Interaction (MCI) method. Moreover if the Radially Uncoupled version of MCI is used, \textitall the radial integrals are simp le products of one dimensional integrals (no correlated integrations at all). The calculations for \mboxH_2^+ including (non-perturbatively) the vibrational and electronic spectra are identical i n structure to those for a He atom, while those for \mboxH_2 to those for Li. Preliminary results for \mboxH_2^+ will be presented.

[J15.80] Precise Measurements of Electric Quadrupole and Dipole Amplitudes in Atomic Thallium

Recently, a 1% measurement of atomic parity nonconservation (PNC) in thallium(P. Vetter et al.) Phys. Rev. Lett. 74, 2658 (1995). has initiated new calculational efforts of the atomic structure of this element. We have undertaken several new high-precision measurements of the atomic structure of thallium which will provide independent tests of these new calculations. In our system, light from a 1.28 \mum external-cavity diode laser excites thallium atoms contained in a sealed, heated quartz cell from the ground 6P_1/2 state to the 6P_3/2 state. The predominantly M1 character of this transition allows measurement of the absorptivity to be used as a direct calibration of atomic number density. Through careful analysis of both absorption and Faraday optical rotation lineshapes of the composite transition, we can determine with high-precision the electric quadrupole amplitude present within the same 1.28 \mum line. Beyond providing a test of atomic theory calculations, knowledge of the E2 amplitude is essential for correct interpretation of PNC in thallium. By comparing the M1 absorption and Faraday rotation to that of a second (frequency-doubled diode) laser tuned to the 378 nm 6P_1/2 - 7S_1/2 electric dipole transition we can similarly determine directly this E1 linestrength with high-precision. Current results will be presented.

[J15.81] Perturbation analysis for 3-D doubly anharmonic sextic potentials.

Approximated eigenvalues for the doubly anharmonic sextic potentials have been obtained. The procedure here followed is a perturbation analysis around the set of parameters where the 3-D radial Schrodinger equation can be solved analytically ( quasi-exact solvable potentials ). A new kind of perturbation up to second order has been performed around this set of parameters, which are those verifying the constrain equation of the exactly solvable potential. The problem is that the knowledge of the ground state function, as well as the excited one for the same parameters, are needed in order to perform the required perturbation analysis. To avoid this difficulty the perturbation analysis has been applied in a different way as it is normally done. Now the analysis utilizes the ground state in a point of the space parameters and the excited states in a different point. Using this procedure the eigenvalues for any value of the parameters have been determined with good accuracy.

[J15.82] A Modified DPM Polarization Potential for e^+--Atom Collisions.

In the original Distributed Positron Model(T.L. Gibson, J. Phys. B) 23, 767-76 (1990). (DPM), nonadiabatic polarization effects due to virtual positronium formation were approximated by treating the positron as a uniform, spherical distribution of positive charge with a fixed radius. However, there is no requirement, other than computational convenience, that constrains this method to such a simple choice for the positive charge distribution. A positive charge distribution which better reflects the distortion of the virtual positronium by the target atom has been developed. The polarization potential obtained with this new charge distribution will be compared with the result from the original method, and scattering cross sections obtained with each potential will be reported.

[J15.83] Molecular Dynamics Simulation of Collisions of Endohedral Carbon Fullerenes

Collisions of (\mboxLa@C_82)^+ with various atomic targets have been investigated using molecular dynamics. We use the many-body carbon potential of Takai et~al.(J.\ Phys.\ Chem.\ \bfseries 94), 4480 (1990), which represents the energy of all of the carbon atoms as a sum over all pairs and triples. The other interactions are modeled using pairwise Morse or Born-Mayer exponential potentials. For each of several selected impact parameters, we calculate the trajectories for several randomly selected initial orientations of the fullerene. Immediately after the collision, the fragmentation pattern of the fullerene is calculated. From this data, the cross section for the production of various fragments is calculated. Initial calculations for Ar projectiles and c.~m.\ energy of 2.8 keV have shown trajectories that lead to the ejection of the La with the loss of only a small number of C~atoms. Further results of the simulation will be compared with recent experimental data.(Lorents et~al., Chem.\ Phys.\ Lett.\ \bfseries 236), 141 (1995).

[J15.84] Chaotic Scattering in Few-Body Systems

We have carried out calculations on chaotic scattering in few-body molecular and atomic systems, including reactive scattering between a fluorine atom and a hydrogen molecule and inelastic scattering between an electron and a helium ion. In both cases we show that self-similar scattering patterns appear which can be correlated to the scattering dynamics. In the case of the helium system, triple-collision orbits and asymmetric-stretch orbit are responsible for the orderly self-similar cusp-shaped patterns. In two-dimensional simulations, the cusp tips reveal further complicated structures, due to frustrated ionizing trajectories. Some of these srtuctures are related to periodic orbits of the helium system. In the case of F + H_2 reaction, fractal scattering patterns are grouped into triplets. We have identified some of these patterns which are strongly influenced by certain periodic orbits.

[J15.85] Inelastic Scattering in Low keV Energy Collisions with Atmospheric Molecules.

This abstract was not submitted electronically.

[J15.86] Observation of Triple Coincidences from the Three-Body, Coulomb-Interacting System H^+ + H^+ + H^-

We have observed for the first time triple coincidences in the long range, Coulomb-interacting system of H^+ + H^+ + H^-. This system is used to study equal mass effects of the three-body Coulomb problem. We produce this system by colliding 4 keV H_3^+ with He. All three fragments from the collision are energy analyzed and angle resolved using a parallel plate energy analyzer equipped with position sensitive detectors. Using coincidence techniques to determine if all three fragments originated from the same H_3^+, we completely determine the dynamics of the H^+ + H^+ + H^- system. Specifically, we will determine in the center of mass frame of the three fragments the total energy available to the fragments, the partitioning of this energy among the fragments, and the correlation angle between the two protons, as well as the inelastic energy loss in the collision of the H_3^+ with the He target. This work is supported by the National Science Foundation under Grant PHY-9419505

[J15.87] Double Dissociative Excitation in H_2^+ - He Collisions

We show that the principal mechanism in H_2^+ - He collisions, leading to the dissociation of H_2^+, is the simultaneous excitation of the projectile and the He target. This is determined from the laboratory energy spectrum of H^+ produced at 0 \deg from collisions of 4 keV H_2^+ with He. During the collision, the H_2^+ has an inelastic energy loss Q. Since many excited states produce H^+ and excitation occurs over a range of internuclear separations, no single value of Q can properly describe the entire energy spectrum. By transforming the entire laboratory energy spectrum to the cm frame of the dissociating H_2^+ for a specific Q and requiring that the resulting spectrum reflect an isotropic distribution of H^+ in the cm frame, we determine a valid range of the H^+ cm energy \epsilon _+ associated with Q. Since \epsilon _+ is half the total energy \epsilon _tot in the cm frame, and \epsilon _tot is the amount of Q in excess of threshold for producing the final state, we can determine the threshold energy for each Q. We find for the majority of H^+, \epsilon _+ \geq 3 eV, and the threshold energy is 36 \pm 2 eV, corresponding to the final state H^+ + H(n\geq 2) + He*. Since this result cannot be explained even qualitatively by the presently accepted models for molecule-atom systems, we submit that the description of inelastic processes is not a simple extension of diatom models.

[J15.88] Projectile-electron loss in low-energy collisions of He^+ with gas targets

The cross section for projectile-electron loss, or stripping, in ion-atom collisions decreases rapidly with decreasing collision energy below 50 keV. We have measured the cross sections for projectile-electron loss for collisions of He^+ ions with various gas targets in the energy range of 10 to 30 keV, extending the energy range of previous measurements. These measurements were conducted at the East Carolina University Accelerator Laboratory. In addition to their fundamental importance, these low-energy stripping cross sections are used as a diagnostic in the ion-ion collisions experiment in development at Oak Ridge National Laboratory.(J.L.~Shinpaugh, F.W.~Meyer, and S.~Datz, Nucl.~Instr.~and Meth.~in Phys.~Res. B99), 198 (1995).

[J15.89] Momentum Distributions of Recoil Ions and Low Energy Electrons Ejected by Fast Projectiles

We have used an imaging technique to measure momentum distributions of low energy continuum electrons and the corresponding recoil ions produced in collisions with fast projectiles. The use of a supersonic gas jet allows the measurement of all three dimensions of momenta for both recoil ions and electrons, as well as determination of the orientation of molecular targets. Precise time of flight measurements of electrons and recoils are made possible by using the KSU LINAC to time-focus projectile ion bunches. Bunch widths of less than 400 psec have been observed. Results for collisions with helium and hydrogen targets are presented. This technique has enabled us to measure the momenta of ``soft'' electrons for a given impact parameter and recoil charge state.

[J15.90] Rotational Energy Transfer of N_2 Determined Using a New Ab Initio Potential Energy Surface

A new N_2-N_2 rigid-rotor surface has been determined using extensive ab initio quantum chemistry calculations together with recent experimental data for the second virial coefficient. Rotational energy transfer is studied using the new potential energy surface (PES) employing the close coupling method below 200 cm^-1 and coupled state approximation above that. Comparing with a previous calculation(W. M. Huo and S. Green, J. Chem. Phys. 104), 7572 (1996). based on the PES of van der Avoird et al.,(A. van der Avoird, P. E. S. Wormer, and A. P. J. Jansen, J. Chem. Phys. 84), 1629 (1986). it is found that the the new PES generally gives larger cross sections for large \DeltaJ transitions, but for small \DeltaJ transitions the cross sections are either comparable or smaller. Correlation between the differences in the cross sections and the two PES will be attempted. The computed cross sections will also be compared with available experimental data.

[J15.91] Double Electron Capture in 90 keV Ne^9+ + He Collisions.

We present high resolution Auger electron spectra of Li-like (ls2 \elln\ell') states in Ne^7+ populated in slow double capture collisions of H-like ions with He. Theoretically we have studied Auger transition energies, withs and branching ratios for nemerouse lithium-like states. Moreover we have computed relativistic coupling effects, which become essential for an understanding of the exitation and deexcitationof Ne^7+ states. In particular we have found evidence for the production of (1s2\elln\ell')^4L states under single collision conditions. This breakdown of LS coupling can cause a violation of the Wigner spin conservation rule, i.e., spin flip processes are possible for systems with Z=10.

[J15.92] Experimental and Theoretical Progress in Spin-polarized Penning Ionization

This poster will describe recent efforts at Illinois State University's Polarized Electron Lab toward theoretical and experimental progress in the study of Penning ionization involving spin-polarized He (2^3S) metastable atoms. Theoretical efforts have centered on providing quantitative models to explain the structure in recent energy-resolved electron polarization measurements resulting from Penning ionization of open-shell targets like O_2 and NO. Several different approaches designed to explain the structure in terms of angular momentum coupling of the participating electron spins lead to some contradictory results. Experimentally, progress results from efforts toward two advances in experimental apparatus. The first is a CW, high-power, lamp-pumped Nd:LMA laser, capable of output power above 2 W in a 2 GHz linewidth (matching the Doppler width of the He metastable atoms). The second is a small, inexpensive, efficient Mott electron polarimeter of cylindrical geometry. Preliminary measurements indicate a scattering efficiency near 10^-2. Details concerning instrumental asymmetry and noise from spurious ions will be reported.

[J15.93] The classical atomic form factor

The general trends exhibited in the variation of the inelastic form factor in collisional transitions nl \rightarrow n^\prime l^\prime, when l^\prime is changed and n,l and n^\prime are kept fixed are explained solely in terms of classical mechanics. The previous quantum mechanical results(M.R.Flannery and K.J.McCann, J.Phys.B, 12), 427 (1979); Astrophys.Journ., 236, 300 (1980) are reproduced from purely classical-mechanics principles. Our conclusions are valid not only for large quantum numbers (which provide the usual classical limit) but also for other cases, which, up to now have only been described by quantal or semi-quantal methods.\footnotemark[2] The interesting trends exhibited in the form factor are directly reflected in theoretical treatments of collisions involving excited atoms.

[J15.94] Barrier Tunneling and Reflection in the Time Domain

We show that for generic barrier potentials which vanish as x\to \pm\infty, the classical trajectories of the semiclassical propagator provide an incomplete description of transmission and reflection, in both the time and energy domains. Classically forbidden processes can dominate classically allowed processes when both are present. Transmission across a barrier in the time domain is classically allowed at any time, but we show the quantum amplitude is dominated by tunneling under the barrier at long times. This has implications for the interpretation and computation of barrier tunneling in the energy domain. The stationary phase time-energy Fourier transform (WKB result) cannot be thought of as an approximation to the (real-time) integral, because of a branch cut in the complex-time plane. In fact, the WKB result arises out of the stationary phase transform of the time-domain tunneling trajectories. Similar observations apply to reflection of amplitude from a barrier where it is at short times that above-barrier quantum reflected trajectories are important.

[J15.95] Multiple Scattering Theory of Two-Dimensional Disordered Billiards

Recent experiments in microwave cavities and semiconductor heterostructures have begun to probe wave-mechanical systems on a scale which can only be understood by considering individual wavefunctions. Therefore, methods for calculating wavefunctions for specific geometries begin to play an important role. Recent work using scattering theory and renormalized multiple-scattering t-matrices will be presented. This will be applied to the calculations of wavefunctions in disordered billiards and compared with microwave experiments.(see A. Kudrolli, et al. Experimental Studies of Chaos and Localization in Quantum Wavefunctions), PRL, vol. 75, no. 5, p. 822-5 Some preliminary statistical analysis (IPR, etc.) of these wavefunctions will be presented. Scattering theory is a natural approach to these problems both because it needs only boundary information \emphand because it easily allows for the phenomonological incorporation of decoherence effects via the free Green's function. Some preliminary results of including the effects of incoherent scattering are presented and compared with experiments in ballistic semiconductor nanostructures.

Part J of program listing