Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session B3: Electron Collisions |
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Chair: Murtadha Khakoo, California State University, Fullerton Room: Burnham Yates Conference Center Olive Branch |
Wednesday, May 18, 2005 10:30AM - 10:42AM |
B3.00001: Electron collisions with Fe peak elements : Application of PRMAT parallel R-matrix codes P.G. Burke, A. Hibbert, B.M. McLaughlin, C.A. Ramsbottom, M.P. Scott, V.M. Burke, C.J. Noble, A.G. Sunderland One of the outstanding problems in electron collisions with atoms and ions is the accurate calculation of cross section data for low ionisation stages of iron peak elements such as iron, nickel and cobalt. There are a number of difficulties which arise from open d-shells in the target states of these ions. Firstly, a large CI expansion is required to adequately represent electron correlation effects within the target ion, and secondly, the open d-shells give rise to a large number of target states, and in turn to a large number of coupled channels. Furthermore, calculations must be carried out over a very fine energy mesh in order to resolve low-lying Rydberg resonances. These difficulties have necessitated a major redevelopment of the standard scalar R-matrix codes to produce the parallel PRMAT codes. These new codes are being applied to the study of electron collisions with a number of ions including FeII, FeIII, FeIV and NiV . Results, illustrating the importance CI in both the target and final continuum states wavefunctions, will be reported at the conference. [Preview Abstract] |
Wednesday, May 18, 2005 10:42AM - 10:54AM |
B3.00002: Direct and indirect relativistic effect on electron scattering from cesium and gold atoms Junfang Gao, Rencheng Shang, Wenning Pang The Stokes parameters, STU parameters and differential cross section (DCS) for electron scattering from a cesium atom are investigated by two kinds of distorted-wave methods. Each of them includes different relativistic effects. Comparing the results with those from the relativistic distorted-wave of the Toronto group, we found that the direct relativistic effect is stronger than the indirect relativistic effect for cesium atom scattering when the incident electron energy is 50 eV. We also noticed that it is the direct relativistic effect that causes the invalidity of the ``fine-structure approximation'' in this case. The Stokes parameters and DCS are also investigated for electron scattering from gold atom when the incident electron energy is 50 eV. On contrary to the case of electron scattering from cesium atom, the indirect relativistic effect is stronger than the direct relativistic effect in the case of electron scattering from gold atom. The qualitative physics interpretation of this phenomenon is that gold atom is heavier than cesium atom, so the electron speed in the gold atom is higher than that in cesium atom. [Preview Abstract] |
Wednesday, May 18, 2005 10:54AM - 11:06AM |
B3.00003: Channel-coupling and second-order effects in electron-impact excitation of Ar(3p) and Ar(3s) Klaus Bartschat, Oleg Vorov Recent experimental studies for electron-impact ionization of argon [1-3] have revealed major problems with numerical approaches, in which the projectile and the ejected electron are described by distorted waves, exchange effects are neglected or approximated by local potentials, and single-configuration descriptions of the initial bound state and the final ionic state are used. We have further developed a hybrid method [4-6], in which the interaction between a ``fast'' projectile and the target is still described perturbatively, up to second-order, while the ejected-electron--residual ion interaction is treated by an $R$-matrix (close-coupling) expansion. This guarantees a proper description of exchange effects for the ``slow'' electron, and it also allows for the use of multi-configuration initial and final target states. The sensitivity of theoretical predictions on the details of the collision model are discussed. \par\noindent [1] M.A. Haynes and B. Lohmann, J. Phys. B {\bf 33} (2000) 4711. \par\noindent [2] M.A. Haynes and B. Lohmann, Phys. Rev. A {\bf 64} (2001) 044701. \par\noindent [3] M. Stevenson {\it et al.}, J. Phys. B {\bf 38} (2005), in press. \par\noindent [4] K. Bartschat and P.G. Burke, J. Phys. B {\bf 20} (1987) 3191. \par\noindent [5] K. Bartschat and P.G. Burke, J. Phys. B {\bf 21} (1988) 2969. \par\noindent [6] K. Bartschat, Comp. Phys. Commun. {\bf 75} (1993) 219. [Preview Abstract] |
Wednesday, May 18, 2005 11:06AM - 11:18AM |
B3.00004: Electron-impact excitation of FeII Oleg Zatsarinny, Klaus Bartschat We have applied the $B$-spline $R$-matrix method~[1,2] to study electron-impact excitation of Fe$^+$ over an energy range from threshold to 5$\,$eV. A major challenge for this astrophysically important collision system is the very complex target structure, with a strong term-dependence in the individual orbitals. Using a multi-configuration Hartree-Fock method with non-orthogonal orbitals allows us to generate individually optimized term-dependent orbitals and thereby limit the number of configurations that need to be kept in both the $N$-electron target and the $(N+1)$- electron collision problems. Overall, our results for individual cross sections and effective collision strengths are in qualitative agreement with the predictions by Ramsbottom {\it et al.}~[3], but we find significant discrepancies in the very complex near-threshold resonance structure. \par\noindent [1] O. Zatsarinny and C. Froese Fischer, J. Phys. B~{\bf 33}, 313 (2000). \par\noindent [2] O. Zatsarinny and K. Bartschat, J. Phys. B~{\bf 37}, 2173 (2004). \par\noindent [3] C.A. Ramsbottom, C.J. Noble, V.M. Burke, M.P. Scott and P.G. Burke, J. Phys. B~{\bf 37}, 3609 (2004). [Preview Abstract] |
Wednesday, May 18, 2005 11:18AM - 11:30AM |
B3.00005: MQDT and wave packet study of dissociative recombination of Ne$_{2}^{+}$ Valery Ngassam, Ousmanou Motapon, Annick Suzor-Weiner, Ann E. Orel We report cross sections for the dissociative recombination of the Ne$_{2}^{+}$ molecular ion following collision with a low energy electron. This reaction plays a very important role in many gaseous discharges. The process is shown to be dominated at low energy by the lowest $^{1,3}\Sigma_{g}^{+}$ resonant dissociative states of Ne$_{2}$. The energy positions and autoionization widths of these doubly excited dissociative states are obtained from electron scattering calculations using the Complex Kohn Variational method while the dissociation dynamics is studied using both multichannel quantum defect theory(MQDT) and wave packet methods. Work supported by the NSF PHY-02-44911 and NATO science program PST.GLG.9794033 [Preview Abstract] |
Wednesday, May 18, 2005 11:30AM - 11:42AM |
B3.00006: Dissociative Electron Attachment to Water: Treatment of $^2B_1$, $^2A_1$, and $^2B_2$ resonances Daniel Haxton, Thomas Rescigno, C. William McCurdy The computational treatment of dissociative electron attachment to gas-phase water via each of the three involved Feshbach resonances, of $^2B_1$, $^2A_1$, and $^2B_2$ symmetry, is discussed, and preliminary results are reported. The behavior of each of these resonances with respect to variations in the nuclear geometry is examined, and several interesting features are discussed. These features include the conical intersection between the $^2A_1$ and $^2B_2$ states and a seam of degeneracy between the $^2B_2$ Feshbach resonance and a $^2B_2$ shape resonance. Seams of degeneracy between metastable states may involve an interesting topology in which the two degenerate states are interchanged by adiabatic following around the seam. This topology is reflected in each of the two seams comprising the $^2A_1$ / $^2B_2$ conical intersection, as well as the $^2B_2$ shape-Feshbach degeneracy. It finds a parallel in the theory of ``hidden crossings'' of bound states at complex nuclear geometries. Preliminary results on dissociative attachment via the Renner-Teller coupled $^2B_1$ ($^2A''$) and $^2A_1$ (1 $^2A'$) states, obtained using the Multi-Configuration Time-Dependent Hartree (MCTDH) method and a local treatment of the nuclear dynamics, are presented and compared to experiment. [Preview Abstract] |
Wednesday, May 18, 2005 11:42AM - 11:54AM |
B3.00007: Electron Impact Vibrational Excitation and Dissociative Attachment of Nitric Oxide Cynthia Trevisan, Karel Houfek, Zhiyong Zhang, Ann Orel, Clyde McCurdy, Thomas Rescigno We present the results of an {\em ab initio} study of elastic scattering and vibrational excitation of NO by electron impact in the low-energy (0-2 eV) region where the cross sections are dominated by resonance contributions. We employ a nonlocal treatment of the nuclear dynamics, which is found to remedy the principal deficiencies of the local complex potential model. We also present cross sections for dissociative electron attachment to NO leading to ground state products, ${\rm O}^-(^2{\rm P}) + {\rm N}(^4{\rm S})$. The calculations show that, while the peak cross sections starting from NO in its ground vibrational state are very small ($\sim 10^{-20}\rm{cm}^2$), the cross sections are extremely sensitive to vibrational excitation of the target and should be readily observable for target NO molecules excited to $\nu = 10$ and above. Preliminary results of low-energy electron scattering by radical CF, which is isoelectronic with NO, will also be discussed. [Preview Abstract] |
Wednesday, May 18, 2005 11:54AM - 12:06PM |
B3.00008: Dissociative attachment and vibrational excitation in low-energy electron-CF$_3$I collisions I. Fabrikant, S. Marienfeld, M. Braun, M. Ruf, H. Hotop Trifluoroiodomethane (CF$_3$I) is a plasma etching gas that provides CF$_3$ radicals and I$^-$ anions through an efficient dissociative electron attachment (DEA) process at near zero electron energies.$^1$ We have carried out a combined experimental and theoretical study of DEA to CF$_3$I at electron energies below 1 eV. Measurements, performed at high resolution with two variants of the laser photoelectron attachment method,$^2$ yielded a joint DEA cross section over the range 1-500 meV, normalized in absolute size to the thermal DEA rate coefficient from swarm data.$^1$ At the onsets for excitation of one and two quanta for the C-I stretching mode $\nu_3$, clear downward cusps are detected. Calculations were performed by using the resonance R-matrix theory and the result for the thermal DEA rate coefficient.$^1$ The calculated DEA cross section shows good overall agreement with the experimental result in both shape and absolute size. In addition, we present theoretical cross sections for vibrational excitation of the $\nu_3$ mode which exhibit strong threshold peaks. $^1$ L. G. Christophorou, J. K. Olthoff, J. Phys. Chem. Ref. Data {\bf 29}, 553 (2000). $^2$ H. Hotop, M.-W. Ruf, M. Allan, I. I. Fabrikant, Adv. At. Mol. Opt. Phys. {\bf 49}, 85 (2003). [Preview Abstract] |
Wednesday, May 18, 2005 12:06PM - 12:18PM |
B3.00009: Electron-impact vibrational excitation of CO$_2$ in a cluster environment Ilya Fabrikant Electron collisions with CO$_2$ clusters are strongly affected by vibrational Feshbach resonances$^1$ which appear due to the long-range polarization interaction between the electron and the cluster. We have developed a theory$^2$ allowing inclusion of this effect in the attachment process and also in vibrational excitation of one molecular unit in the cluster environment. We predict a rich resonance structure in vibrational excitation of Fermi polyads representing mixtures of symmetric stretch and bending vibrations. A zero-range-potential model has been developed to investigate a possibility of simultaneous vibrational excitation of two or more molecular units. $^1$ H. Hotop, M.-W. Ruf, M. Allan, I. I. Fabrikant, Adv. At. Mol. Opt. Phys. {\bf 49}, 85 (2003). $^2$ I. I. Fabrikant and H. Hotop, Phys. Rev. Lett., to be published. [Preview Abstract] |
Wednesday, May 18, 2005 12:18PM - 12:30PM |
B3.00010: Theoretical and Experimental study of the Spin Asymmetry for Electron Impact Ionization of Xenon D.H. Madison, A. Prideaux, J. Lower, S. Bellm, R. Panajotovic, E. Weigold The study of differential cross sections for atomic ionization by electron impact has provided a very sensitive and useful tool for determining the importance of different types of physical effects in a collision process. If the spin polarization of the electron beam is also known, an additional level of sensitivity is provided. It was recently shown, for example, that spin dependent measurements for atomic ionization provided a very sensitive test of exchange effects between the projectile electron and atomic electrons which could not be observed in measurements without polarized beams. Although the DWBA (Distorted Wave Born Approximation) results are in reasonably good agreement with spin-asymmetry measurements, there are significant cases of disagreement. New and improved spin- asymmetry measurements for electron impact ionization of xenon have been performed and these measurements will be compared with DWBA and 3DW (3-Body Distorted Wave) calculations. The importance of different scattering mechanisms will be examined. [Preview Abstract] |
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B3.00011: The Triple Differential Cross Section and Interference Effects of Electron-Impact Ionization of Molecules Junfang Gao, Don H. Madison, Jerry Peacher The Highest Occupied Molecular Orbital (HOMO) is important for chemical reactions and some biological processes. Electron impact ionization of molecules (e,2e) is an effective probe for studying the important molecular interactions. Recently double slit type interference effects for diatomic molecules have also received considerable attention. For high incident-energy (keV range) electron-impact ionization, the Plane Wave Impulse Approximation (PWIA), which was developed in the 1970's, provides a good understanding of the experimental differential cross section data. However for lower incident-electron energies, no accurate theory exists to describe the ionization process. We have developed a Three-body Distorted Wave (3DW) approach which treats all continuum electrons as distorted waves and which also includes the final state electron-electron interaction (correlation) to all orders of perturbation theory. In addition, we have developed the Distorted Wave Impulse Approximation (DWIA) which is a distorted wave version of the PWIA. Both theories will be used to study triple differential cross sections and interference effects for electron-impact ionization of hydrogen and nitrogen molecules. Some results for large molecules will also be reported. [Preview Abstract] |
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