Bulletin of the American Physical Society
65th Annual Gaseous Electronics Conference
Volume 57, Number 8
Monday–Friday, October 22–26, 2012; Austin, Texas
Session SR3: Electron Collisions with Atoms and Molecules |
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Chair: Klaus Bartschat, Drake University Room: Classroom 202 |
Thursday, October 25, 2012 3:30PM - 4:00PM |
SR3.00001: Low-energy electron interactions with water and methanol Invited Speaker: Leigh Hargreaves Interactions of electrons with small, highly polar molecules form a crucial basis for understanding cellular damage caused during radiation therapy of cancers. Recent studies of elastic scattering from water by Khakoo \textit{et al.} [1], and electronically inelastic scattering by Thorn \textit{et al.} [2], have revealed serious discrepancies between experimental and theoretical determinations of the cross sections for these processes, and in some cases even between experimental values. In this presentation, new data for both elastic and electronic excitation of water are presented which aim to resolve some of these discrepancies. We will also consider excitation cross sections for a more complicated target, methanol. The measured methanol data are the first such data reported. \\[4pt] [1] M.A. Khakoo \textit{et al.}, \textit{Phys. Rev. A}, \textbf{78}, 052710 (2010)\\[0pt] [2] P.A. Thorn \textit{et al.}, \textit{J. Phys. B: At. Mol. Phys.}, \textbf{40}, 697 (2007) [Preview Abstract] |
Thursday, October 25, 2012 4:00PM - 4:15PM |
SR3.00002: Electron Collision Processes with Carbon Dioxide: Resolving Long-Standing Paradoxes T.N. Rescigno, D.J. Haxton, C.W. McCurdy The principal features of low-energy electron-CO$_2$ collisions have been known and studied for over forty years. The scattering is characterized by a rapid rise in the total cross section below 1 eV, anomalous threshold behavior for excitation of symmetric stretch and bending vibrational modes, resonant vibrational excitation near 4 eV with weak ``boomerang'' structure in the excitation cross sections and dissociative electron attachment cross sections leading to CO + O$^-$ which peak near 4 eV and 8 eV and have angular distributions which show large deviations from axial recoil. The nuclear dynamics associated with all these features is intrinsically polyatomic in nature and cannot be described with one-dimensional models. The present study provides a consistent description of all these phenomena and resolves a number long-standing paradoxes and misconceptions found in the extant literature. [Preview Abstract] |
Thursday, October 25, 2012 4:15PM - 4:30PM |
SR3.00003: Accuracy of OAMO (Orientation Averaged Molecular Orbital) approximation for calculating electron-impact ionization cross sections for molecules Adam Upshaw, Ben Payne, James Colgan, Don Madison We have been using the M3DW (molecular 3-body distorted wave) approximation plus the OAMO (orientation averaged molecular orbital) to calculate cross sections for electron-impact ionization of molecules. The approximation yielded good agreement with experiment for H2 and reasonable agreement for N2. However, the agreement was not that good for H2O, CH4, and larger molecules so the important question concerns if the disagreement is a result of the OAMO approximation or a problem with the theoretical approach. Consequently we have modified our computer codes to perform a proper average over molecular orientations. M3DW results both with and without making the OAMO approximation will be compared with experimental measurements. [Preview Abstract] |
Thursday, October 25, 2012 4:30PM - 5:00PM |
SR3.00004: Cross Sections for Electron-impact Excitation of Electronic States in Atoms and Molecules -Application Examples of the BE$f$-scaling model in Optically-allowed Transitions Invited Speaker: Hidetoshi Kato The differential cross section (DCS) and integral cross section (ICS) measurements of electron-impact electronic excitation for noble gases were reported by many groups. Unfortunately, despite all these endeavours, if we were to characterize the level of agreement between these studies, then we could only conclude that it remains ``patchy'' at best. Hence, we report measurements of DCS and ICS for electron-impact excitation of the lowest electronic states in noble gases (Ne, Ar, Kr and Xe), and compare with results from a scaled Born cross section (BE$f$-scaling). We have previously found for the He atom and a number of molecules, including H$_{2}$, O$_{2}$, CO, H$_{2}$O, CO$_{2}$, N$_{2}$O and C$_{6}$H$_{6}$ that the BE$f$-scaling approach, for calculating ICSs for dipole-allowed electronic-state transitions, can lead to a very accurate description of the various scattering processes from threshold to 2000 eV [1]. Exceptions to this general statement have only been found in those cases where resonance effects due to the temporary capture of the incident electron by the target and contamination from an accidentally degenerate or near-degenerate triplet state have arisen. Our results for noble gases will be presented in detail at the conference.\\[4pt] [1] H. Kato et al., NIFS Research Report, NIFS-DATA-108, 1 (2009). [Preview Abstract] |
Thursday, October 25, 2012 5:00PM - 5:15PM |
SR3.00005: Relativistic convergent close-coupling calculation of the spin polarization of electrons scattered elastically from zinc and mercury Christopher Bostock, Dmitry Fursa, Igor Bray We present spin asymmetry parameters (Sherman functions) for elastic electron scattering on zinc and mercury atoms calculated using the relativistic convergent close-coupling (RCCC) method. The Zn and Hg atoms are each modeled as two active electrons above a Dirac-Fock core. Three key features of the RCCC method are critical: (1) an {em ab initio} treatment of spin via the Dirac equation, (2) a unitary treatment of the scattering process, and (3) correct antisymmetrization of the total wave function. There is excellent agreement between the RCCC results and experiment for the case of Hg across a wide range of energies, and similarly there is excellent agreement between RCCC results and experiment for Zn across the range of energies where $3d^{10}$ core excitation levels do not appear. The results are relevant in light of the recent controversial claim by Williams \textit{et al.} [Phys.~Rev.~A 85(2011)022701] that relativistic scattering theories do not account for spin properly during electron scattering on quasi two-electron targets such as Zn and Hg; the claim is made that a geometric ``Berry'' phase is required to augment fundamental scattering theories. [Preview Abstract] |
Thursday, October 25, 2012 5:15PM - 5:30PM |
SR3.00006: Quantum Control of Diatomic Molecular Vibration States Using Space-Time Discretization Charles Weatherford, Xingjun Zhang Molecular evolution in an external field can be efficiently calculated by using our Space-Time algorithm (STA), which deploys a basis set in space and time and turns Quantum Mechanical initial-value problems into a set of simultaneous algebraic equations and can be used to simulate the control of the Quantum states of a molecule. The external field must be optimized so as to restrict the search space to those parameter values characterizing the field, which are available in experiments. A generalized evolution strategy is employed which provides a technique for parameter optimizations. We have developed a parallel algorithm which implements an evolution strategy using the STA to optimize the external field intensity (laser amplitude) so as to drive the molecule from an initial vibrational state to a desired vibrational state. We optimize the molecular evolution procedure in two ways: a step-by-step optimization and a closed-loop optimization. The method is applied to NO$^+$. The simulation has been done using 5 vibrational states. [Preview Abstract] |
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