Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session J3: Focus Session: Electron-Atom Collisions |
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Chair: A. Dasgupta, Naval Research Laboratory Room: TELUS Convention Centre Glen 201-203 |
Thursday, June 7, 2007 1:30PM - 2:06PM |
J3.00001: High-Precision Calculations for Electron-Impact Excitation and Ionization of Complex Atoms Invited Speaker: While electron collisions with the valence electrons of light (quasi-)one- and (quasi-)two-electron atoms, such as H, He, the alkalis, and the alkali-earth elements, can now be handled very accurately by various theoretical methods (convergent close-coupling, $R$-matrix with pseudo-states, exterior complex scaling, or time-dependent close-coupling), the situation is much less satisfactory for complex open-shell targets such as Mo or Fe, or for collisions involving excitation or ionization of inner shells in the heavy noble gases. In recent years, our group has further developed an alternative version of the $R$-matrix (close-coupling) method, using a $B$-spline basis with non-orthogonal sets of term-dependent orbitals [1-3]. This method allows us to generate target descriptions of unprecedented accuracy in collision calculations, which turn out to be of critical importance for accurate results, particularly for resonances in the near-threshold region. In addition, a hybrid method, combining a second-order distorted-wave method for a fast ionizing projectile with a convergent $R$-matrix with pseudo-states approach for the initial bound state and the ejected electron [4], has been very successful in treating ionization of heavy noble gases~[5] and simultaneous ionization-excitation of He [6]. Examples will be presented at the conference. \par\noindent [1] O. Zatsarinny and C. Froese Fischer, J. Phys. B {\bf 33} (2000) 313. [2] O. Zatsarinny and K. Bartschat, J. Phys. B {\bf 37} (2004) 2173. [3] O. Zatsarinny, Comp. Phys. Commun. {\bf 174} (2006) 273. [4] Y. Fang and K. Bartschat, J. Phys. B {\bf 34} (2001) L19. [5] K. Bartschat and O.~Vorov, Phys. Rev. A {\bf 72} (2005) 022708. [6] S. Bellm, J. Lower, and K. Bartschat, Phys. Rev. Lett. {\bf 96} (2006) 223201. [Preview Abstract] |
Thursday, June 7, 2007 2:06PM - 2:18PM |
J3.00002: Correlated R-matrix theory of electron scattering: A coupled-cluster approach Chiranjib Sur, Anil Pradhan, P. Sadayappan Study of electron scattering from heavy atoms/ions not only demands high speed computing machines but also improved theoretical descriptions of the relativistic and correlation effects for the target atoms/ions as well. We will give an outline of the coupled-cluster R-matrix (CCRM) theory to incorporate the effect of electron correlation through coupled-cluster theory (CCT), the size extensive and one of the most accurate many body theories which is equivalant to an all-order many-body perturbation theory (MBPT). General theoretical formulation of CCRM and the computational implementation using the high level Mathematica style language compiler known as Tensor Contraction Engine (TCE) will be presented. Electronic structure calculations using CCT involve large collections of tensor contractions (generalized matrix multiplications). TCE searches for an optimal implementation of these tensor contraction expressions and generates high performance FORTRAN code for CCT. We will also comment on the interfacing of TCE generated code with the Breit-Pauli R-matrix code to make a next generation CCRM software package. This theoretical formulation and the new sets of codes can be used to study electron scattering / photoionization in heavy atomic systems where relativistic and electron correlation effects are very important. [Preview Abstract] |
Thursday, June 7, 2007 2:18PM - 2:30PM |
J3.00003: Hybrid Theory of Electron-Hydrogen Scattering Anand Bhatia I report on a study of electron-hydrogen scattering, using a combination of a modified method of polarized orbitals and the optical potential formalism which does not require projection operators. The calculation is restricted to S-wave scattering in the elastic region, where the correlation functions are of Hylleraas type. It is found that the phase shifts are not significantly affected by the modification of the target function by a method similar to the polarized orbitals and they are close to the phase shifts calculated by Bhatia and Temkin [Phys. Rev. A 64, 032709-1 (2001)]. This indicates that the correlation function is general enough to include the target distortions (polarization) in the presence of the incident electron, except for scattering lengths for which inclusion of polarization is essential for precision results. Results for the phase shifts, obtained in the present hybrid formalism, are rigorous lower bounds to the exact phase shifts. [Preview Abstract] |
Thursday, June 7, 2007 2:30PM - 2:42PM |
J3.00004: Electron-impact ionization of atomic ions in the B isonuclear sequence Julian Berengut, Stuart Loch, Michael Pindzola, Connor Ballance, Don Griffin Electron-impact ionization cross sections for several atomic ions in the B isonuclear sequence are calculated using both perturbative and non-perturbative theoretical methods. A distorted wave calculation in a mixed $V^N$/$V^{N-1}$ potential for neutral B exhibits a large shape resonance in the 2s and 2p subshell ionization cross-sections near their thresholds, which is not found in an all $V^{N-1}$ potential distorted-wave calculation. The time-dependent close-coupling and $R$-matrix with pseudo-states methods provide a check for the different distorted wave methods. We also plan to present ionization cross section results for B$^{+}$, B$^{2+}$, and B$^{3+}$. [Preview Abstract] |
Thursday, June 7, 2007 2:42PM - 3:18PM |
J3.00005: Recent Progress in Electron-Atom Scattering Invited Speaker: The application of multi-parameter data collection techniques to electron-atom collision-experiments allows statistically significant results to be obtained for weak physical effects [1]. In addition to improved count rates, if the spin projection of the primary electron is determined, the roles of electron exchange and relativity in the scattering process can be highlighted. Examples of recent measurements from our laboratories will be discussed in the context of work from other groups. These will include benchmark measurements on the electronic excitation of helium employing the time-of-flight technique [2]. The technique allows inelastic cross sections to be accurately placed on an absolute scale by normalization to well-established elastic cross sections. Measurements on the (e,2e) ionization/excitation of helium through the application of energy-dispersive toroidal-analyzers will also be discussed [3]. The results provide a stringent test to theory and indicate the strengths and limitations of state-of-the art calculations in describing the Coulomb 4-body problem. Finally I will describe results from (e,2e) measurements on argon [4] and xenon [5] targets performed with spin polarized electrons which probe the many-body nature of electron exchange-scattering. \newline [1] J. Ullrich \textit{et al}, Rep. Prog. Phys. \textbf{66} (2003) 1463. \newline [2] M. Lange \textit{et al, }J. Phys. B: At. Mol. Opt. Phys. \textbf{39} (2006) 4179. \newline [3] S. Bellm, J. Lower and K. Bartschat, Phys. Rev. Lett. \textbf{96}, 223201 (2006). \newline [4] S. Bellm, J. Lower, Marco Kampp and Colm T. Whelan, J. Phys. B: At. Mol. Opt. Phys. \textbf{39 }(2006) 4759. \newline [5] R. Panajotovic, J. Lower, E. Weigold A. Prideaux and D. H. Madison, Phys. Rev. A \textbf{73}, 52701, (2006). [Preview Abstract] |
Thursday, June 7, 2007 3:18PM - 3:30PM |
J3.00006: Multi-Dimensional Momentum Images of Electron-Impact Induced Ionization of Atoms and Molecules J. Ullrich, A. Dorn, M. Duerr, N. Haag, Ch. Dimopoulou A dedicated Reaction Microscope was developed that enables to measure the complete momentum vectors of electrons and ions emitted in ionizing collisions of electrons with atoms and molecules. Realizing a special geometrical design we are able to access very low projectile energies, to detect the scattered projectile electron in addition to all target fragments, and to observe ionic fragments from molecular dissociation. Selected recent results will be highlighted: For single ionization of He by 1 keV and 105 eV electrons the 3D electron emission patterns show structures outside the scattering plane, so far not been observed in conventional experiments.\footnote{M. D\"{u}rr et al., Phys. Rev. Lett. \underline {96}, 243202 (2006)} Studies for double ionization of He close to the threshold provide detailed insight into the behaviour of four strongly interacting continuum particles. Finally, pioneering (e,2e) results for single ionization of oriented H$_{2}$ molecules are presented. [Preview Abstract] |
Thursday, June 7, 2007 3:30PM - 3:42PM |
J3.00007: Out-of-plane ($e,2e$) experiments on an autoionizing resonance using 488 eV incident energy electrons. B.A. deHarak, N.L.S. Martin Over the past several years there have been a variety of kinematically complete experiments with both coplanar and out-of- plane geometry involving charged particle impact ionization of a variety of atomic targets. Examples where data were obtained using COLTRIMS spectrometers include single ionization of helium by ions and electrons. An example where data were obtained using a more traditional spectrometer involved single ionization of magnesium by electron impact. Each of these studies has shown that while theoretical descriptions of coplanar experiments tend to be very good, there are large discrepancies in the description of out-of-plane experiments, providing ample motivation for further experimental studies. We have begun to perform kinematically complete out-of-plane experiments on helium using a traditional ($e,2e$) spectrometer modified for out-of-plane operation. These experiments cover all 2$\pi$ radians of a plane that includes the momentum transfer direction and is perpendicular to the scattering plane. An overview of the apparatus will be presented as will preliminary results showing the angular distributions for direct ionization and autoionization via He $2s2p~^1P$. [Preview Abstract] |
Thursday, June 7, 2007 3:42PM - 3:54PM |
J3.00008: Electron Scattering From Laser Excited Ba and Yb Atoms Peter Zetner, Jeff Hein Inelastic and elastic electron scattering out of the excited states of Ba and Yb has been studied at low collision energies. We present measurements (at 10 eV and 20 eV collision energies) of differential cross sections and orientation parameters for elastic scattering out of the 6 $^{1}$P$_{1}$ and 5 D levels of Ba and for inelastic scattering out of the 6 $^{3}$P$_{1}$ level of Yb to higher lying $^{3}$D and $^{3}$S levels. Collision studies are carried out using a momentum selected incident electron beam with momentum resolved scattered electron detection and excited atomic target populations generated by resonant laser radiation. To a good approximation, Ba and Yb are heavy two-electron systems comprising filled, approximately inert cores ([Xe] for Ba and [Xe]4f$^{14}$ for Yb) with two-electron 6s$^{2}$ valence configurations (in the ground state). Theoretical determinations of low-energy electron scattering parameters for these atoms, based on convergent close coupling and first order perturbative formalisms, have met with success in many cases. Such calculations have been applied to inelastic scattering from 6S, 5D and 6P target states in Ba and the 6S ground state in Yb (refs 1,2). Extension of previous experimental investigations to the new scattering processes described in this work will further test the efficacy of available theoretical methods. 1. I. Bray et al. J.Phys.B:At.Mol.Opt.Phys. \textbf{35} R117 (2002) 2. B Predojevic et al. J. Phys. B: At. Mol. Opt. Phys. \textbf{38} 3489 (2005) [Preview Abstract] |
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