Bulletin of the American Physical Society
61st Annual Gaseous Electronics Conference
Volume 53, Number 10
Monday–Friday, October 13–17, 2008; Dallas, Texas
Session CT2: Electron/Photon Interactions with Molecules |
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Chair: Chetan G. Limbachiya, P.S. Science and H.D. Patel Arts College, Kadi - Gujarat, India Room: Salon A-D |
Tuesday, October 14, 2008 10:00AM - 10:30AM |
CT2.00001: Electron- and Photon-Induced Fragmentation Dynamics in Simple Atoms and Molecules Invited Speaker: Advanced imaging techniques such as COLTRIMS, coupled with high-resolution electron- and new X-ray photon sources, have triggered a new class of kinematically complete experiments that probe the details of electron- and nuclear dynamics in simple atoms and small molecules. The interpretation of these experiments presents a formidable challenge for contemporary theory, often involving fully differential cross section and excited-state reaction dynamics calculations that require precise, non-perturbative quantum mechanical methodology. I will illustrate this topic with several examples in which state-of-the-art {\em ab initio} theory can shed light on experiments which probe electron and/or nuclear fragmentation dynamics in simple systems. The examples will include sequential and non-sequential two-photon double ionization of helium, diffraction effects in high-energy photo-double ionization of H$_2$, core-hole localization in molecular nitrogen, inner-valence shell fragmentation of carbon monoxide and electron-induced three-body breakup of water. [Preview Abstract] |
Tuesday, October 14, 2008 10:30AM - 11:00AM |
CT2.00002: Electron- Molecule Collisions: Recent results for ``large'' polyatomic molecules Invited Speaker: Electron interactions with molecular targets, particularly large polyatomic systems, pose a significant challenge for scattering theory and experiment alike. Many targets, such as those of relevance to radiation damage in the body, are difficult to model as they are generally large, polar systems, which are also highly polarisable. They can also be difficult to produce experimentally as they are either liquids or powders at room temperature. This talk will discuss some recent results for elastic scattering and vibrational excitation of molecules such as formic acid, tetrahydrofuran and 3-hydroxy tetrahydrofuran -- all of which can be considered as useful analogues to studies of the larger molecular constituents of DNA. Work done in collaboration with Violaine Vizcaino, James Sullivan, Michael Brunger, Jason Roberts, Vincent McKoy and Carl Winstead. [Preview Abstract] |
Tuesday, October 14, 2008 11:00AM - 11:15AM |
CT2.00003: GEC Student Award for Excellence Finalist: Fluorescence Polarization of H$_{2}$, D$_{2}$, and N$_{2}$ Molecules Excited by Polarized Electron Impact J.W. Maseberg, T.J. Gay We report relative Stokes parameters for both molecular and dissociated atomic transitions in H$_{2}$, D$_{2}$, and N$_{2}$ excited by spin-polarized electrons. In the case of molecular transitions, we find that values of the normalized circular polarization fraction, P$_{3}$/P$_{e}$, for light emitted in the direction of electron spin polarization are non-zero for H$_{2}$ and D$_{2}$, but consistent with zero for N$_{2}$. Specifically, we have succeeded in rotationally isolating the H$_{2}\,d\,{ }^3\Pi _u \to a\,{ }^3\Sigma _g^+ $Q(1) 601.8 nm transition. It exhibits relatively large near-threshold values for P$_{3}$/P$_{e}$ ($\sim $17{\%}). This is in contrast to the null measurements for the N$_{2}\,C\,{ }^3\Pi _u \to B\,{ }^3\Pi _g $ 380.5 nm and N$_{2}^{+}\,B\,{ }^2\Sigma _u^+ \to X\,{ }^2\Sigma _g^+ $ 391.4 nm transitions. For the case of fluorescence from dissociated atomic fragments we observe non-zero circular polarization values for H, D, and N. The near-threshold P$_{3}$/P$_{e}$ values observed for the Balmer-alpha H and D 656.3 nm transitions are nearly equivalent ($\sim $8{\%}). For the N$\,({ }^3P)3p\to ({ }^3P)3s$ 824.2 nm transition we find P$_{3}$/P$_{e}$ values of approximately -7{\%}. This surprising negative sign indicates that the atomic fragment is spinning the ``wrong way,'' i.e. in a direction opposite that of the incident electron responsible for the molecular dissociation. [Preview Abstract] |
Tuesday, October 14, 2008 11:15AM - 11:30AM |
CT2.00004: Double Photoionization of H$_2$: Double Slit Interference? Daniel Horner, Shungo Miyabe, Felipe Morales, Fernando Martin, Thomas Rescigno, C. William McCurdy Using the method of exterior complex scaling and the finite element discrete variable representation, we are able to compute accurate time-independent wave functions for the double photoionization of H$_2$. Together with integral amplitude methods, these accurate wave function solutions allow us to calculate converged differential cross sections. We present the double differential cross section at photon energies between 130 eV and 240 eV. At these energies, recent experiments have observed angular distributions that were interpreted as double slit interference of the electrons ejected from near the two nuclei. With our theoretical methods, we are able to examine this question further and will offer an alternate interpretation based on the coherent mixture of parallel and perpendicular polarization in the circularly polarized light. [Preview Abstract] |
Tuesday, October 14, 2008 11:30AM - 11:45AM |
CT2.00005: Low Energy Elastic Scattering from Water Murtadha A. Khakoo, John Muse, Helen Silva, Maria Cristina A. Lopes Differential electron scattering cross-sections have been measured for elastic scattering from water molecules. The method of relative flow, but with an aperture replacing traditional gas collimating structures. In addition, the gas source was moved to determine the background scattering contribution The measurements were taken at incident energies of 1eV to 100eV and scattering angles of 5$^{\circ}$ to 130$^{\circ}$. Comparisons with theory and former experiments will be presented. [Preview Abstract] |
Tuesday, October 14, 2008 11:45AM - 12:00PM |
CT2.00006: Ly-$\alpha $ and H-$\alpha $ Optical Excitation Functions Resulting From Photodissociation of H$_{2}$ and D$_{2}$ T.J. Gay, E. Litaker, J.R. Machacek, J.E. Furst, A.L.D. Kilcoyne, F. Martin, K.W. McLaughlin, J.L. Sanz-Vicario The production of Ly-$\alpha $ and H-$\alpha $ radiation following photodissociation of H$_{2}$ plays a key role in astrophysical plasmas associated with planetary aurorae and new-star formation. However, these very basic cross sections are not well known at this time, due to the poor agreement between the extant experimental data and the best available theory [1]. We will present recent data we have taken at the Lawrence Berkeley Laboratory Advanced Light Source (ALS) for these excitation functions with H$_{2}$, HD, and D$_{2}$ targets with incident photon energies between 24 and 70 eV. We will discuss possible systematic errors in the available data sets, including those due to incident photon spectral impurity, target contamination, cascading, background gas quenching, and other target-pressure-dependent effects. [1] See J.D.Bozek \textit{et alii}, J.Phys.B\textbf{ 39}, 4871 (2006), and references therein. [Preview Abstract] |
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