Bulletin of the American Physical Society
60th Gaseous Electronics Conference
Volume 52, Number 9
Tuesday–Friday, October 2–5, 2007; Arlington, Virginia
Session GW2: Electron and Positron: Transport and Annihilation |
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Chair: Rainer Johnsen, University of Pittsburgh Room: Doubletree Crystal City Crystal Ballroom B |
Wednesday, October 3, 2007 8:00AM - 8:30AM |
GW2.00001: Current Issues in Electron and Positron Transport Theory Invited Speaker: In this paper we review the current status of transport theory for low energy electrons or positrons in gases, in the context of both kinetic theory and fluid modelling. In particular, we focus on the following issues: (i) Muliterm vs two-term representation of the velocity distribution function in solution of Boltzmann's equation; (ii) the effect of non-conservative collisions (attachment, ionization, positron annihilation) on transport properties; (iii) the enduring electron- hydrogen vibrational cross section controversy and possible implications for the Boltzmann equation itself; (iv) closure of the fluid equations and the heat flux \textit{ansatz}; and (v) correct use of swarm transport coefficients in fluid modelling of low temperature plasmas. Both hydrodynamic and non-hydrodynamic examples will be given, with attention focussed on the Franck-Hertz experiment, particularly the ``window'' of fields in which oscillations of transport properties are produced, and the way in which electric and magnetic fields combine to affect transport properties. \newline \newline In collaboration with co-authors Z. LJ. Petrovi\'c, Institute of Physics Belgrade, and R.D. White, James Cook University. [Preview Abstract] |
Wednesday, October 3, 2007 8:30AM - 9:00AM |
GW2.00002: Resonances and Bound States in Positron Annihilation on Molecules Invited Speaker: Positron annihilation is important in such diverse areas as study of metabolic processes in the human brain and the characterization of materials. Annihilation on molecules has been a subject of keen interest for decades. In particular, annihilation rates can be orders of magnitude greater than those expected for simple collisions. Recent results put our understanding of many aspects of this long-standing problem on a firm footing. We now understand that the annihilation proceeds by vibrational Feshbach resonances (VFR). A prerequisite for the existence of these VFR is that the positron binds to the target. The annihilation energy spectra provide the best measures to date of positron binding energies. Predictions of a new theory of VFR-enhanced annihilation in small molecules (methyl halides) [1] show excellent, quantitative agreement with experiment. New data and analyses for larger molecules (e.g., hydrocarbons with more than two carbon atoms) show that annihilation rates depend strongly on the number of vibrational degrees of freedom but, surprisingly, only weakly on positron binding energy. This places important constraints on theories of annihilation in these molecules. Results for second bound (i.e., positronically excited) states and overtone and combination-mode VFR, as well as outstanding questions, will also be discussed. This work is done in collaboration with Jason Young. \newline \newline [1] G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. 97, 193201 (2006). [Preview Abstract] |
Wednesday, October 3, 2007 9:00AM - 9:30AM |
GW2.00003: Theory of Positron Annihilation on Molecules Invited Speaker: Recently there has been a rapid progress in understanding enhanced positron annihilation on polyatomic molecules. Building on the hypothesis about the role of vibrational Feshbach resonances [1] and their first observations in alkanes [2], positron binding energies have been determined for many molecules [3]. Also, first calculations of resonant annihilation have been performed and showed excellent agreement with the measured annihilation rates in methyl halides [4]. I will review the current theoretical understanding of the two annihilation mechanisms, direct and resonant. While the complete problem of positron-molecule annihilation is very complex, its various aspects can be modeled by relatively simple means. Thus, by using zero-range potentials we can study the scaling of the positron binding energy with the size of the molecule. For small polyatomics with infrared active modes (e.g., methyl halides), a complete calculation of resonant annihilation can be done, in good agreement with experiment. Applying this theory to other molecules highlights the role of overtones and combination vibrations, and ultimately, intramolecular vibrational redistribution. \begin{enumerate}\setlength{\itemsep}{-3pt}\setlength{\itemindent}{-12pt} \item G. F. Gribakin, Phys. Rev. A {\bf 61}, 022720 (2000). \item S. J. Gilbert {\em et al.}, Phys. Rev. Lett. {\bf 88}, 043201 (2002); L. D. Barnes, S. J. Gilbert, and C. M. Surko, Phys. Rev. A {\bf 67}, 032706 (2003). \item L. D. Barnes, J. A. Young, and C. M. Surko, Phys. Rev. A {\bf 74}, 012706 (2006). \item G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. {\bf 97}, 193201 (2006). \end{enumerate} [Preview Abstract] |
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