Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session U6: Electron-Atom and Positron-Atom Collisions |
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Chair: Klaus Bartschat, Drake University Room: Nittany Lion Inn Boardroom II |
Saturday, May 31, 2008 8:00AM - 8:12AM |
U6.00001: Coupled-cluster based R-matrix codes (CCRM): Recent developments Chiranjib Sur, Anil K. Pradhan We report the ongoing development of the new coupled-cluster R-matrix codes (CCRM) for treating electron-ion scattering and radiative processes within the framework of the relativistic coupled-cluster method (RCC), interfaced with the standard R-matrix methodology. The RCC method is size consistent and in principle equivalent to an all-order many-body perturbation theory. The RCC method is one of the most accurate many-body theories, and has been applied for several systems. This project should enable the study of electron-interactions with heavy atoms/ions, utilizing not only high speed computing platforms but also improved theoretical description of the relativistic and correlation effects for the target atoms/ions as treated extensively within the RCC method. Here we present a comprehensive outline of the newly developed theoretical method and a schematic representation of the new suite of CCRM codes. We begin with the flowchart and description of various stages involved in this development. We retain the notations and nomenclature of different stages as analogous to the standard R-matrix codes. [Preview Abstract] |
Saturday, May 31, 2008 8:12AM - 8:24AM |
U6.00002: Full-Relativistic B-Spline R-Matrix Calculations for Electron Collisions with Gold Atoms. Oleg Zatsarinny, Klaus Bartschat, Charlotte Froese \hbox{Fischer} We have extended the $B$-spline $R$-matrix (close-coupling) method~[1] to fully account for relativistic effects in a Dirac-Coulomb formulation. The newly developed computer code has been applied to electron-impact excitation of the $\rm (5d^{10}6s)^2S_{1/2} \to (5d^{10}6p)^2P_{1/2,3/2}$ and $\rm (5d^{10}6s)^2S_{1/2} \to (5d^{9}6s^2)^2D_{5/2,3/2}$ transitions in Au. Our numerical implementation of the close-coupling method enables us to construct term-dependent, non-orthogonal sets of one-electron orbitals for the bound and continuum electrons. This is a critical aspect in the present problem, especially for the 5d and the 6s orbitals. Furthermore, strong core-polarization effects can be accounted for {\it ab initio\/} rather than through a semi-empirical and local model potential. Our results will be compared with recent experimental data~[2] and predictions from other theoretical approaches~[3]. [1]~O.~Zatsarinny, Comp. Phys. Commun. {\bf 174}, 273 (2006). [2]~M.~Maslov, P.J.O. Teubner, and M.J.~Brunger, private communication (2008). [3]~D.V.~Fursa, I. Bray, and R.P. McEachran, private communication (2008). [Preview Abstract] |
Saturday, May 31, 2008 8:24AM - 8:36AM |
U6.00003: Electron Impact Ionization of Xenon Atomic Ions John A. Ludlow, Stuart D. Loch, Michael S. Pindzola Excitation-autoionization features are studied in electron collisions with low-charged atomic ions in the Xenon isonuclear sequence. Calculations are made using semi-relativistic multi-configuration distorted-wave codes recently adapted for massively parallel computers. Theoretical predictions are compared with recent crossed-beams experiments for Xe$^+$ through to Xe$^{10+}$. The studies are extended to higher charged atomic ions of Xenon to map out the influence of radiation damping on excitation-autoionization features. [Preview Abstract] |
Saturday, May 31, 2008 8:36AM - 8:48AM |
U6.00004: Differential Cross Sections for Electron-Impact Single and Double Ionization of Helium Michael S. Pindzola, John A. Ludlow, Francis J. Robicheaux, James P. Colgan Correlated quantal three-body and four-body dynamics are studied in the electron-impact single and double ionization of Helium. Calculations are made using non-perturbative 6D and 9D time-dependent close-coupling codes adapted for massively parallel computers. Theoretical calculations of triple differential cross sections for single ionization of Helium and pentuple differential cross sections for double ionization of He are compared with recent low-energy reaction microscope experiments. [Preview Abstract] |
Saturday, May 31, 2008 8:48AM - 9:00AM |
U6.00005: Electron impact autoionization in the presence of a laser field B.A. deHarak, K.D. Wells, K.B. MacAdam, N.L.S. Martin We have begun to perform the first electron impact autoionization experiments in the presence of a laser field. Laser assisted electron collisions are a poorly understood process. A theoretical discussion of electron impact ionization in the presence of a laser field was published as long ago as 1988,\footnote{C. J. Joachain et al., Phys. Rev. Lett. 61, 165 (1988)} but the first experimental results were only published in 2005. \footnote{C. H\"{o}hr {\em et al.} Phys. Rev. Lett. 94, 153201 (2005)} Distinct differences in the ionization process between the field free (laser off) and field assisted (laser on) cases were found. The results were in poor agreement with quantum calculations but could (oddly enough) be explained with a simple classical model. Our experiments have the advantage that autoionizing levels provides energy markers: the absorption or emission of a photon is expected to shift the resonance position in the ejected electron spectrum by the photon energy. It is easier to detect the presence of such sideband peaks than it is to observe quantitative shifts in the energy onset of traditional simultaneous electron-photon excitation (SEPE) experiments. We will provide details of our experimental apparatus, and provide a progress report on these experiments. [Preview Abstract] |
Saturday, May 31, 2008 9:00AM - 9:12AM |
U6.00006: Energy of the quasi-free electron in low density Ar and Kr: Extension of the local Wigner-Seitz model Xianbo Shi, Luxi Li, Gina Moriarty, Cherice Evans, Gary Findley We present new measurements of the perturber induced shift of the CH$_3$I ionization energy at low perturber number densities and analyze these data within the local Wigner-Seitz model previously used at high density. We compare the local Wigner- Seitz model with the scattering approach developed by Fermi, and modified by Alekseev and Sobel'man, and show that the local Wigner-Seitz model extends smoothly into the low density region and accurately predicts the temperature dependence of the shift in ionization energy. \newline \emph{The experimental measurements reported here were performed at the University of Wisconsin Synchrotron Radiation Center (NSF DMR-0537588). This work was supported by grants from the Petroleum Research Fund (45728-B6), from the Professional Staff Congress -- City University of New York (60074-34 35) and from the Louisiana Board of Regents Support Fund (LEQSF(2006-09)-RD- A33).} [Preview Abstract] |
Saturday, May 31, 2008 9:12AM - 9:24AM |
U6.00007: High resolution positron scattering from helium James Sullivan, Peter Caradonna, Adric Jones, Stephen Buckman, Casten Makochekanwa High resolution positron scattering measurements have commenced at the Australian Positron Beamline Facility. A positron beam with an energy resolution of 60 meV has been used to make total cross section measurements of positron scattering from helium below the positron formation threshold, in particular in the region of previously reported resonance features between 1 and 3 eV scattering energy [1]. No evidence of any resonances was found in the measurements which agree well with theoretical calculations, as well as previous experiments made with a substantially worse energy resolution. Measurements of low energy elastic differential cross sections will also be presented and compared to theoretical calculations, with the goal of establishing a benchmark for positron scattering. [1] Karwasz et al., NIM B \textbf{240}, (2005) 666 [Preview Abstract] |
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