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 Q3: Focus Session: Electron-Molecule Collisions |
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Chair: T. Rescigno, Lawrence Berkeley National Laboratory Room: TELUS Convention Centre Macleod B |
Friday, June 8, 2007 1:30PM - 2:06PM |
Q3.00001: Low Energy Electron Molecule Collisions Invited Speaker: Low energy electron molecule collisions are important in many technological, environmental and biological processes. Electrons are known to be the `drivers' of technological processes based around gas discharges, from lights and lamps to surface processing plasmas. In such environments electrons are responsible for much of the vibrational and electronic excitation that results in photon emissions, for the formation of long-lived metastable species, and for the production of highly reactive free radicals through processes such as dissociative attachment. Electron collisions also play an important role in our atmosphere and those of all planets and stars. In recent years low energy electrons have also been shown to play a vital role in cell and tissue damage caused by ionizing radiation. One of the key goals for collision physics, both experiment and theory, is to provide accurate, absolute cross sections for such low energy charged particle collisions - elastic scattering, vibrational excitation, near-threshold electronic excitation. This talk will focus on some recent advances in these areas. This work is done in collaboration with James Sullivan, Violaine Vizcaino, Stan Newman, Julian Lower, Subhendu Mondal, Chris Colyer, Michael Brunger, Todd Maddern and Leigh Hargreaves. [Preview Abstract] |
Friday, June 8, 2007 2:06PM - 2:18PM |
Q3.00002: Theoretical and Experimental Study of Out-of-Plane Cross Sections for Electron Impact Ionization of He and H2 Ola Al-Hagan, D.H. Madison, Christian Kaiser, Andrew Murray In recent years, sharp disagreement between theory and experiment has been found for heavy particle ionization of He for electrons ejected out of the scattering plane. On the other hand, good agreement was found in the scattering plane. The lack of agreement between experiment and theory for out-of-plane has been attributed to a double scattering mechanism where the projectile first `hits' the electron and then scatters off the nucleus. If these effects are important for a He nucleus, they should be even more important for a H2 nucleus. We will report theoretical and experimental fully differential cross section (FDCS) results for electron impact ionization of both He and H2. Electron ejection both in the scattering plane and out-of- plane will be examined. [Preview Abstract] |
Friday, June 8, 2007 2:18PM - 2:30PM |
Q3.00003: Elastic Differential Cross sections for Electron Scattering from Polyatomic Molecules -- An Accurate, but Novel Application of the Relative Flow Technique, Using a Moveable Aperture Source of Gas Atoms Kyle Keane, Shayne Cairns, Colin Campbell, Murtadha A. Khakoo We have very recently developed a powerful and novel method to measure elastic differential scattering cross sections (DCS) from gaseous targets, without having to take into consideration the mean-free path condition which means that our method can be applied without knowledge of the molecular diameters of the gases used. This is possible by taking advantage of the cosine angular distribution of gas produced by a thin aperture source. Preliminary tests with N$_{2}$ and C$_{2}$H$_{4}$ have produced results in excellent agreement with past measurements of these gases, and over what seems to be a wide range of gas flow conditions. In addition, our results show no degradation of angular resolution in our DCS values even at very low incident energies (presently we have covered 5eV to 30eV). We hope to provide data for other polyatomics at the meeting. [Preview Abstract] |
Friday, June 8, 2007 2:30PM - 2:42PM |
Q3.00004: Electron Impact Induced VUV Emissions from Nitrous Oxide C.P. Malone, P.V. Johnson, J.W. McConkey, J.M. Ajello, I. Kanik Emissions in the VUV have been probed for transitions from various excited electronic states (e.g., N$^{\ast}$ (3s $^{4}$P)) resulting from electron impact of nitrous oxide (N$_{2}$O). The experimental setup consists of a 0.2m spectrometer and an electron collision chamber. The electron beam (with energy spread of $\sim$1eV) was magnetically collimated with an axially symmetric magnetic field and accelerated to a given kinetic energy. A Faraday cup was used to monitor the beam current and to eliminate any back-scattered electrons. Photons emitted by electron impact excited target molecules and excited atoms following dissociative excitation, were dispersed by the spectrometer and detected with a channel electron multiplier coated with CsI. Measured emission spectra and emission cross sections as functions of incident energy will be presented. [Preview Abstract] |
Friday, June 8, 2007 2:42PM - 3:18PM |
Q3.00005: Intrinsically Polyatomic Phenomena in Electron-Molecule Scattering Invited Speaker: Resonant collisions of electrons with molecules are one of the most efficient pathways for the transfer of energy from electronic to nuclear motion. As the field of electron-molecule scattering developed, both experimentally and theoretically, the phenomena of vibrational excitation and dissociative attachment were first understood for diatomics, and it seemed natural to extend that understanding to polyatomic molecules using one-dimensional or single-mode models of the nuclear motion. However a series of recent experimental measurements of these phenomena in small polyatomic molecules have proven to be uninterpretable in terms of atomic motion with a single degree of freedom. Why do resonant collisions of electrons with CO$_2$ preferentially excite particular members of nearly degenerate groups (Fermi polyads) of vibrational levels? Why does dissociative attachment of electrons to H$_2$O at 6.5 eV produce primarily H$^-$ + OH while attachment at 11.5 eV produces mostly O$^-$ + H$_2$? How does a $\pi$ shape resonance in electron scattering from formic acid produce dissociative attachment products of an obviously different symmetry? The answers to these questions involve multidimensional nuclear motion, multiple potential surfaces for the metastable anions, and conical intersections. Using $\textit{ab initio}$ methods of electron scattering and bound state quantum chemistry we can generate those multidimensional potential surfaces for small molecules. With wave packet propagation on those coupled surfaces we can elucidate the mechanisms and reveal how they are probed in the experiments. It appears that these complexities are a general feature of electronic collisions with polyatomic molecules, and that they may hold the key to understanding how collisions of low-energy electrons produced by ionizing radiation damage biological molecules. [Preview Abstract] |
Friday, June 8, 2007 3:18PM - 3:30PM |
Q3.00006: Resonances and threshold effects in low-energy electron collisions with methyl halides Ilya Fabrikant, Gordon Gallup Cross sections for elastic and inelastic electron collisions with CH$_3$X (X=Cl, Br, I) molecules are calculated. For the lowest partial wave the resonance R-matrix theory, and for the higher partial waves the theory of scattering by dipolar plus polarization potential, are used. It is shown that the rotationally elastic scattering amplitude for a polar molecule in the fixed-nuclei approximation is logarithmically divergent for the forward direction, and a new closure formula is derived to speed up the convergence at small angles. The dipole moment as a function of the C--X distance is modeled semiempirically. This is supplemented by {\it ab initio} calculations of the dipole moment function for CH$_3$Br using the multi-configurational valence bond method. The results for scattering cross sections show pronounced features caused by vibrational Feshbach resonances and threshold cusps. The features are most noticeable at the $v=6$, 7, and 8 thresholds in CH$_3$Cl, at the $v=3$ and 4 thresholds in CH$_3$Br and at the $v=1$ threshold in CH$_3$I. The authors are grateful to H. Hotop for many stimulating discussions. [Preview Abstract] |
Friday, June 8, 2007 3:30PM - 3:42PM |
Q3.00007: The Role of Nuclear Motion in the Photo-Double Ionization of Molecular Hydrogen Thomas Rescigno, Daniel Horner, Wim Vanroose, Fernando Martin, C. William McCurdy We explain the origin of recently observed variations with internuclear distance (R) of the fully differential cross sections for double ionization of aligned H$_2$ by a single photon. Using the results of converged numerical solutions of the Schr\"odinger equation, we show that these variations arise primarily from pronounced differences in the R-dependence of the parallel and perpendicular components of the ionization amplitude. We also predict that R-dependences should be readily observable in the asymmetry parameter for photo-double ionization, even in experimental measurements that are not differential in the energy sharings between ejected photo-electrons. [Preview Abstract] |
Friday, June 8, 2007 3:42PM - 3:54PM |
Q3.00008: The role of nuclear dynamics in molecular ionization processes produced by synchrotron radiation and ultrashort pulses Fernando Martin The important role of nuclear dynamics in molecular ionization processes produced by synchrotron radiation and ultrashort pulses will be demonstrated using accurate ab initio theoretical calculations that account for all electronic and vibrational degrees of freedom. Results for electron angular distributions from fixed-in-space H$_{2}$ molecules will be presented. For photon energies of a few hundreds of eV, it is shown that, for molecules oriented parallel to the polarization direction, the angular patterns reveal a complex nodal structure, while for molecules oriented perpendicularly, typical Young's double-slit interferences are observed. These patterns change dramatically as the molecule vibrates. For photon energies of $\sim $ 30 eV, it is shown that the emission of a photoelectron with subsequent dissociation of the remaining H$_{2}^{+}$ fragment shows no symmetry with respect to the ionic H$^{+}$ and neutral H atomic fragments. This lack of symmetry results from the entanglement between symmetric and antisymmetric H$_{2}^{+ }$states caused by autoionization. The dependence on pulse parameters of multiphoton ionization of H$_{2}$ by ultrashort pulses will also be analyzed. Refs: A. Palacios et al, Phys. Rev. Lett. \textbf{96}, 173201 (2006); J. Fern\'{a}ndez et al, Phys. Rev. Lett. \textbf{98, }043005 (2007); F. Mart\'{\i}n et al, Science \textbf{315}, 630 (2007). [Preview Abstract] |
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