Bulletin of the American Physical Society
2006 59th Annual Gaseous Electronics Conference
Tuesday–Friday, October 10–13, 2006; Columbus, Ohio
Session QR2: Electron and Positron Collisions |
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Chair: Murtadha Khakoo, California State University, Fullerton Room: Holiday Inn Salon B |
Thursday, October 12, 2006 10:00AM - 10:30AM |
QR2.00001: Electron Collisions with Large Molecules Invited Speaker: In recent years, interest in electron-molecule collisions has increasingly shifted to large molecules. Applications within the semiconductor industry, for example, require electron collision data for molecules such as perfluorocyclobutane, while almost all biological applications involve macromolecules such as DNA. A significant development in recent years has been the realization that slow electrons can directly damage DNA. This discovery has spurred studies of low-energy collisions with the constituents of DNA, including the bases, deoxyribose, the phosphate, and larger moieties assembled from them. In semiconductor applications, a key goal is development of electron cross section sets for plasma chemistry modeling, while biological studies are largely focused on understanding the role of localized resonances in inducing DNA strand breaks. Accurate calculations of low-energy electron collisions with polyatomic molecules are computationally demanding because of the low symmetry and inherent many-electron nature of the problem; moreover, the computational requirements scale rapidly with the size of the molecule. To pursue such studies, we have adapted our computational procedure, known as the Schwinger multichannel method, to run efficiently on highly parallel computers. In this talk, we will present some of our recent results for fluorocarbon etchants used in the semiconductor industry and for constituents of DNA and RNA. In collaboration with Carl Winstead, California Institute of Technology. [Preview Abstract] |
Thursday, October 12, 2006 10:30AM - 11:00AM |
QR2.00002: Electron Collisions with Atoms and Molecules Relevant to Industrial, Planetary and Astrophysical Plasmas. Invited Speaker: Collision processes involving electrons with neutral atoms and molecules are among the principal mechanisms for energy exchange in industrial and naturally occurring plasmas. In many of these plasmas, large populations of low energy, secondary electrons are generated. Such secondary electrons are very effective in many of the relevant collision processes because both the cross sections and the electron energy distributions peak at low energies. In this talk, a series of experiments involving electron collisions with atomic oxygen, vibrationally excited molecular hydrogen and molecular nitrogen will be presented. Electron collisions with these species are of particular importance in planetary plasmas as demonstrated by, for example, the auroral phenomena at Earth and Jupiter, the electroglow of Jupiter, Saturn and Uranus, and UV emission from Earth, Titan and Triton, respectively. Further, such interactions are important in plasma processing of textile materials, nitrogen gas lasers, and fusion plasmas. The experiments to be presented involve two main experimental techniques (1) electron energy loss spectroscopy and (2) electron impact induced emission spectroscopy. [Preview Abstract] |
Thursday, October 12, 2006 11:00AM - 11:30AM |
QR2.00003: Electron-atom collision theory and applications Invited Speaker: During the last decade computational methods for electron-atom collisions have undergone major advances. In developing the convergent close-coupling (CCC) method the fundamental goal has been to have a theory which was valid at all projectile energies and for all atomic transitions of interest. Currently the CCC method has been shown to be particularly successful in reproducing experimental data for atomic transitions that are dominated by one-electron processes. This is the case for the lighter atoms such as H, He, He$^+$, Li, and Na. Much of our effort in recent years has gone towards generalising the possible targets to incorporate heavier and more complex atoms which are important in various applications. In particular, we have considered Zn, Ba, and Hg for the lighting industry. Currently we are extending the method to inert gases such as Ne and Ar. In the talk we will review the CCC method and its applications, and also discuss the possible future directions. [Preview Abstract] |
Thursday, October 12, 2006 11:30AM - 11:45AM |
QR2.00004: Resonances in positron-molecule annihilation J.A. Young, C.M. Surko The positron (antiparticle to the electron) has unique interactions with matter. In atoms and molecules, it is attracted to the electrons and can annihilate to form gamma radiation. Using monoenergetic positrons from a trap-based beam, we have measured the positron-on-molecule annihilation rates as a function of positron energy [1,2]. In many molecules, we observe greatly increased annihilation at a fixed energy below the vibrational mode energies.This enhancement is due to quasi-bound positron states populated via vibrational Feshbach resonances (VFR). The resonant annihilation peaks grow exponentially with molecular size. In this paper, we discuss the latest data on positron capture and annihilation in small molecules, and we present new data on the effect of molecular temperature on annihilation rates. Finally, we relate the data to models of VFR and discuss the role of intramolecular vibrational energy distribution in the annihilation process. \\ ~[1] S. J. Gilbert, \textit{et al.}, \textit{Phys. Rev. Lett}., \textbf{88}, 043201 (2002).\\ ~[2] L. D. Barnes, \textit{et al.}, \textit{Phys. Rev. A} \textbf{67}, 032706 (2003). [Preview Abstract] |
Thursday, October 12, 2006 11:45AM - 12:00PM |
QR2.00005: Elastic Scattering of Electrons by Small and Large Molecules. D.W. Spieker, Junfang Gao, J.L. Peacher, D.H. Madison Theoretical calculations for elastic electron-molecule collisions will be presented for incident electron energies in the intermediate to low energy range. There exists a fair amount of experimental data for which there are no theoretical calculations for comparison in the 10-500 eV energy range. We will present differential cross section results for molecular hydrogen (H$_{2})$, molecular nitrogen (N$_{2})$, and trifluoromethane (CHF$_{3})$. We have used the program called General Atomic and Molecular Structure System, or GAMESS, to generate molecular orbitals. We use these to determine a potential interaction energy for the electron-molecule system. The distorted wave Born approximation (DWBA) was used to calculate the differential cross sections. The theoretical differential cross section results will be compared with experimental results. [Preview Abstract] |
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