Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session SR2: Electron and Positron Collisions with Atoms |
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Chair: Thomas M. Miller, Boston College Room: Saratoga Hilton Ballroom 2 |
Thursday, October 22, 2009 10:00AM - 10:30AM |
SR2.00001: Electron impact induced light emission from zinc atoms Invited Speaker: Experimental studies of electron impact excitation of zinc atom are rare, primarily due to experimental difficulties. However, zinc is an interesting target because of possible applications in light sources. Also, due to its position in periodic table, zinc is an interesting case for the fundamental understanding of momentum couplings and the role of electron correlations in complex metal atoms. Recent experimental investigations have indicated the existence of highly correlated scattering mechanisms via formation of negative ion resonances and Post Collision Interaction (PCI) in the decay of autoionizing states. These can significantly modify energy dependence of the emission cross sections at low impact energies and the studies of photon emission offer a sensitive way to investigate electron correlations. Specifically, in the lowest autoionizing region of zinc, i.e. between 10 and 15 eV, both the cross sections and polarization of emitted light are affected by the formation of short lived negative ions and PCI effects. These are associated with excitation of one of the sub-valence 3d electrons and complex correlations between inner 3d and outer excited electrons in the target and also with the slow electron released into continuum, need to be included in modeling. Also the scattering of the spin polarized electrons has shown significant spin effects when excitation proceeds via negative ion resonances. Emission cross sections and comparison with theory would be discussed at the conference. [Preview Abstract] |
Thursday, October 22, 2009 10:30AM - 10:45AM |
SR2.00002: Benchmark calculations for electron collisions with mercury Oleg Zatsarinny, Klaus Bartschat We have applied our recently developed fully relativistic Dirac $B$-spline $R$-matrix (DBSR) code~[1] to calculate electron scattering from mercury atoms. Results from a \hbox{36-state} close-coupling calculation~[2] are compared with numerous experimental benchmark data for angle-integrated and angle-differential cross sections, as well as spin-asymmetry, spin-polarization, and electron-impact coherence parameters. We generally obtain much better agreement with experiment than previous distorted-wave and close-coupling attempts. The results are believed to be particularly accurate in the low-energy (below 10~eV) near-threshold regime and hence represent a significant improvement over frequently used older datasets for modeling of low-temperature plasmas containing mercury.\\[4pt] [1] O.~Zatsarinny and K.~Bartschat, Phys. Rev. A {\bf 77}, 062701 (2008).\\[0pt] [2] O.~Zatsarinny and K.~Bartschat, Phys. Rev. A {\bf 79}, 042713 (2009). [Preview Abstract] |
Thursday, October 22, 2009 10:45AM - 11:00AM |
SR2.00003: Complex $q$ parameters for helium $L=0,1,2$ autoionizing levels N.L.S Martin, B.A. deHarak, K. Bartschat We recently reported\footnote{B.A. deHarak, K. Bartschat, and N.L.S. Martin, {\em Phys. Rev. Lett.}, {\bf 100}, 063201 (2008).} out-of-plane (e,2e) experiments on He autoionization. The data were presented as angular distributions of ejected electrons from the three autoionizing levels $^1S$, $^1D$, and $^1P$ and exhibited two well known features, the binary and recoil peaks. It was found that the recoil peak (relative to the binary peak) could be accurately reproduced by a second order distorted wave Born calculation using the $R$-matrix with pseudo-states approach, but not by the equivalent {\it first order} calculation, which underestimated the size of the recoil peak. It was also found that a plane wave Born approximation calculation could reproduce the results, but only if anomalously large values of Fano $q$-parameters were assumed. We will present an analysis of the first and second order calculations in terms of Fano $q$ parameters. We find that for the first order calculations the $q$ parameters are essentially real, but for the second order calculations they are complex, quantities. The $^1D_2$ parameters are particularly striking in this respect. This work was supported by the U.S. NSF under Grants~No.~PHY-0855040 (NLSM) and PHY-0757755 (KB). [Preview Abstract] |
Thursday, October 22, 2009 11:00AM - 11:15AM |
SR2.00004: Differential cross sections for modeling of noble gas plasmas Allan Stauffer, Robert McEachran Differential cross sections are required to model the 3D diffusion of electrons in a gas under the influence of electromagnetic fields. In a low temperature plasma containing a noble gas elastic scattering from the neutral atoms is an important process governing this diffusion even at energies above the inelastic thresholds. We have calculated the phase shifts at such energies using our optical potential method [1] which takes account of open inelastic channels. This method yields much more accurate data than potential scattering calculations using only real potentials over a wide range of scattering energies. We will present analytic fits to our results so that cross sections may be calculated for arbitrary scattering energies.\\[4pt] [1] S. Chen, R. P. McEachran and A. D. Stauffer, J.Phys. B 41, 025201 (2008) [Preview Abstract] |
Thursday, October 22, 2009 11:15AM - 11:30AM |
SR2.00005: Accurate atomic data for xenon: energy levels, oscillator strengths, and electron collision cross sections Klaus Bartschat, Oleg Zatsarinny We have applied our recently developed fully relativistic Dirac $B$-spline $R$-matrix (DBSR) code~[1] to calculate the atomic structure (energy levels and oscillator strengths) as well as electron scattering from xenon atoms. Results from a \hbox{31-state} close-coupling model for the excitation function of the metastable \hbox{${\rm (5p^5 6s)}~J=0,2$} states show excellent agreement with experiment~[2], thereby presenting a significant improvement over the most sophisticated previous Breit-Pauli calculations~[3,4]. The same model is currently being used to calculate electron-impact excitation from the metastable $J=2$ state. The results will be compared with recent experimental data~[5] and predictions from other theoretical models~[6,7]. Our dataset is an excellent basis for modeling plasma discharges containing xenon.\\[0pt] [1]~O.~Zatsarinny and K.~Bartschat, Phys. Rev. A {\bf 77} (2008) 062701.\\[0pt] [2]~S.~J. Buckman {\it et al}., J. Phys. B {\bf 16} (1983) 4219.\\[0pt] [3]~A.~N.~Grum-Grzhimailo and K.~Bartschat, J. Phys. B {\bf 35} (2002) 3479.\\[0pt] [4]~M.~Allan {\it et al}., Phys. Rev. A {\bf 74} (2006) 030701(R).\\[0pt] [5]~R.~O. Jung {\it et al}., Phys. Rev. A {\bf 72} (2005) 022723.\\[0pt] [6]~R. Srivastava {\it et al}., Phys. Rev. A {\bf 74} (2006) 012715.\\[0pt] [7]~J.~Jiang {\it et al}., J. Phys. B {\bf 41} (2008) 245204. [Preview Abstract] |
Thursday, October 22, 2009 11:30AM - 11:45AM |
SR2.00006: Electron excitation into the Xe($5p^57p$) configuration from the ground and $1s_5$ ($J$=2) metastable level R.O. Jung, John B. Boffard, L.W. Anderson, Chun C. Lin We have measured electron excitation cross sections into the $3p_x$ (Paschen's notation for $5p^57p$) levels of Xe by passing an electron beam through a static gas target (for ground state excitation) or an atomic beam emerging from a hollow cathode (for excitation from metastable levels) and detecting fluorescence from the decay of excited atoms. Excitation cross sections out of the ground level, especially at high energies, exhibit moderate dependence on gas pressure due to cascades from resonant levels. For excitation out of the $1s_5$ ($J$=2) metastables into the $3p_6$, $3p_7$, $3p_8$, and $3p_9$ levels ($J$=2,1,3,2), which conform to optical selection rules, the excitation functions demonstrate a variety of shapes ranging from an expected broad maximum to a sharp peak riding on a structure of intermediate width. The cross section data for the various $3p_x$ levels are interpreted in terms of oscillator strengths as well as angular momentum coupling of the states involved. The $3p_x \to 1s_y$ emissions are prominent components of Xe plasmas, making our measured cross sections valuable for optical plasma diagnostics. [Preview Abstract] |
Thursday, October 22, 2009 11:45AM - 12:00PM |
SR2.00007: Positron Interactions with Rare Gas Atoms A. Jones, P. Caradonna, C. Makochekanwa, J. Sullivan, D. Slaughter, S. Buckman, D. Mueller, S. Bellm, B. Lohmann We present results for low energy positron scattering from the rare gas atoms He, Ne and Ar. A high resolution, trap-based positron beam has been used for these measurements which encompass absolute cross sections for total scattering, positronium formation, electronic excitation and total ionization at energies between 0.5 and 60 eV. Where possible we make comparison with previous experiments and with contemporary scattering theory for the total scattering and positronium formation cross sections. The excitation measurements have focused on the 2$^{1}$S,P states of helium at energies between threshold and 40 eV, and they represent the first state selective measurements of these two excited states. A key motivation of the ionization measurements is to study the near-threshold `Wannier' regime, and we will present measurements that map the energy dependence of the ionization cross section within 2 eV of threshold. [Preview Abstract] |
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