Bulletin of the American Physical Society
61st Annual Gaseous Electronics Conference
Volume 53, Number 10
Monday–Friday, October 13–17, 2008; Dallas, Texas
Session LW2: Electron/Positron Atom Collisions |
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Chair: M.A. Khakoo, California State University at Fullerton Room: Salon A-D |
Wednesday, October 15, 2008 1:30PM - 2:00PM |
LW2.00001: Calculation of Atomic Data for Plasma Applications Invited Speaker: Accurate and complete datasets for atomic structure and collision parameters are important for many applications in plasma physics, in particular the modeling and diagnostic of discharges. In recent years, we have developed a general computer code [1], based on the close-coupling expansion, that allows for the calculations of such data both for the target structure (energy levels and oscillator strengths) and electron collisions with atoms and ions. The general idea behind such calculations will be discussed, as well as the expected accuracy of the results, current limitations of theory, recent developments including a fully relativistic version [2], and planned extensions for the future. The method will be illustrated with oscillator strengths and electron collision cross sections for the noble gases He-Xe, alkali metals such as a Na and Cs, ions such as FeII, and very heavy targets such as Au and Hg. \newline [1] O. Zatsarinny, Comp. Phys. Commun. {\bf 174} (2006) 273. \newline [2] O. Zatsarinny and K. Bartschat, Phys. Rev. A {\bf 77} (2008) 062701. [Preview Abstract] |
Wednesday, October 15, 2008 2:00PM - 2:15PM |
LW2.00002: Positron Scattering from Neon Al Stauffer, Robert McEachran, James Sullivan, Casten Makochekanwa, Peter Caradonna, Adric Jones, Daniel Slaughter, Stephen Buckman A joint experimental and theoretical investigation of low-energy positron-neon scattering has been carried out. The experimental studies have used a high-resolution, trap-based beam and scattering cell to obtain absolute cross sections for total scattering, total elastic scattering and positronium formation. The experimental and theoretical total elastic cross section are compared from near threshold up to the positronium formation threshold at $\sim $15 eV. Theoretically, the elastic differential cross section has also been calculated up to 50 eV including, where appropriate, absorption effects via an ab initio absorption potential. [Preview Abstract] |
Wednesday, October 15, 2008 2:15PM - 2:30PM |
LW2.00003: Low-Energy Electron Collisions with Copper and Gold Atoms Klaus Bartschat, Oleg Zatsarinny We have extended the $B$-spline $R$-matrix (close-coupling) method~[1] to fully account for relativistic effects in a Dirac-Coulomb formulation~[2]. The computer code was applied to electron-impact excitation of the $\rm (3d^{10}4s)^2S_{1/2} \to (3d^{10}4p)^2P_{1/2,3/2}$ and $\rm (3d^{10}4s)^2S_{1/2} \to (3d^{9}4s^2)^2D_{5/2,3/2}$ transitions in Cu and the corresponding transitions $\rm (5d^{10}6s)^2S_{1/2} \to (5d^{10}6p)^2P_{1/2,3/2}$ and $\rm (5d^{10}6s)^2S_{1/2} \to (5d^{9}6s^2)^2D_{5/2,3/2}$ 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 problems, especially for the outermost d and s orbitals. Furthermore, core-polarization effects are accounted for {\it ab initio\/} rather than through a model potential. Our results will be compared with recent experimental data~[3] and predictions from other theoretical approaches~[4]. [1]~O.~Zatsarinny, Comp. Phys. Commun. {\bf 174}, 273 (2006). [2]~O.~Zatsarinny and K. Bartschat, Phys. Rev. A {\bf 77}, 062701 (2008). [3]~M.~Maslov, P.J.O. Teubner, and M.J.~Brunger, Phys. Rev. A {\bf 77}, in press (2008). [4]~D.V.~Fursa, I. Bray, and R.P. McEachran, private communication (2008). [Preview Abstract] |
Wednesday, October 15, 2008 2:30PM - 2:45PM |
LW2.00004: Electron-photon coincidence experiment in full scattering angle range Lukasz Klosowski, Mariusz Piwinski, Dariusz Dziczek, Katarzyna Pleskacz, Stanislaw Chwirot Electron impact coherence parameters (EICP) have been measured with coincidence technique for various collisional systems since 1970s. Large discrepancies occurring in various theoretical predictions could not be resolved because of lack of experimental data for large scattering angles. Experiments that provide data for the largest scattering angles have not been carried out for seemingly simple reason -- finite dimensions of electron beam sources and energy analyzers. Such measurements are possible if electron trajectories are suitably modified by magnetic field (Rev. Sci. Instrum. 67 (1996) 2372). It has been also shown recently that the magnetic angle changer (MAC) could be used in EICP measurements (Meas. Sci. Technol. 18 (2007) 3801, J. Phys. B: At. Mol. Opt. Phys. 41 (2008) 055202). The principles of MAC operation and the design of the device used in coincidence experiment are presented together with new experimental data on EICPs for 100 eV e-He 2$^1$P$_1$ impact excitation for full scattering angle range. [Preview Abstract] |
Wednesday, October 15, 2008 2:45PM - 3:00PM |
LW2.00005: Electron scattering from atomic gallium: data for plasma physics modeling D.V. Fursa, I. Bray Accurate electron-atom collision data is of primary importance for plasma physics modeling. Often the number of required transitions is very large and realistically can only be provided by theoretical calculations of electron-atom collisions. With development of sophisticated scattering methods, such as Convergent Close Coupling (CCC) method and R-matrix (RM) method, accurate collision data can be obtained for large number of scattering systems. Recent developments in CCC and RM methods allow for accurate calculations of target atoms which are strongly affected by relativistic effects. Gallium is one such target, with the ground state being a P-state that is strongly affected by spin-orbit interaction resulting in fine-structure splitting of 0.1 eV. We have performed fully-relativistic CCC calculations of electron scattering from gallium and compared with the available experimental and theoretical data. We find large discrepancies with the collision data used for the modeling of gallium-iodine positive column discharge plasma, which recently attracted substantial attention as a possible candidate for designing nontoxic (mercury-free) light sources. [Preview Abstract] |
Wednesday, October 15, 2008 3:00PM - 3:15PM |
LW2.00006: Absolute angle-differential cross sections for near-threshold electron-impact excitation of neon Michael Allan, Kai Franz, Hartmut Hotop, Oleg Zatsarinny, Klaus Bartschat Absolute angle-differential cross sections for excitation of neon atoms to the four (2p$^{5}$3s) and selected (2p$^{5}$3p) and (2p$^{5}$3d) levels have been determined as a function of electron energy up to 3.5$\,$eV above threshold at scattering angles of $0^\circ$, $45^\circ$, $90^\circ$, $135^\circ$ and $180^\circ$. Some cross sections were also recorded as function of scattering angle from $0^\circ$ to $180^\circ$, which was possible through the use of a ``magnetic angle changer''. Comparison of the experimental data with theoretical predictions based on Breit-Pauli \hbox{$B$-spline} \hbox{$R$-matrix} calculations shows overall good agreement regarding both the absolute values and the details of numerous resonant features. Some discrepancies remain, however, whose possible origin will be discussed at the conference. [Preview Abstract] |
Wednesday, October 15, 2008 3:15PM - 3:30PM |
LW2.00007: Electronic Excitation of low-lying Excited States of Argon and N2 by Electron Impact Subhendu Mondal, Julian Lower, Stephen Buckman, Gustavo Garcia We have used a low energy, time-of-flight (ToF) electron spectrometer to measure absolute cross sections for the near-threshold excitation of low-lying levels of N2 and for the 4s and 4p manifolds of states in argon. The measurements cover an energy range from 12.5 -- 15 eV and an angular range from 50-125\r{ }. The absolute values of the cross sections are obtained by reference to the absolute elastic scattering cross section. The ToF technique has the advantage that the transmission of the energy analyser is independent of energy and it thus removes one of the major uncertainties involved in such measurements with conventional electrostatic spectrometers. [Preview Abstract] |
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