Bulletin of the American Physical Society
63rd Annual Gaseous Electronics Conference and 7th International Conference on Reactive Plasmas
Volume 55, Number 7
Monday–Friday, October 4–8, 2010; Paris, France
Session LW3: Electron and Positron Collisions with Molecules |
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Chair: Murtadha Khakoo, California State University, Fullerton Room: Petit Amphitheatre |
Wednesday, October 6, 2010 4:00PM - 4:30PM |
LW3.00001: Gaseous positronics: collisions and transport of positrons in gases Invited Speaker: While it may still be too early to call for a Gaseous Positronics Conference, as a result of the increased activity in the field of positron induced atomic and molecular physics, cross section sets that are sufficiently complete to model low energy positron transport in gases could be developed for a number of gases including Ar, N2, H2 and CF4. Calculations of the transport coefficients of low energy positrons ($<$10 eV) in gases have shown some of the standard features similar to electrons, but also some new kinetic phenomena that were not observed for electrons. These include the negative differential conductivity (NDC) for the bulk component of the drift velocity, while there is not even a favourable situation for the NDC for the flux component. A similar effect is observed as a large anisotropy of the bulk diffusion. Most recently, in the studies of positron transport in ExB fields, it was found that direction of drift may be quite different, by as much as 70 degrees, between the flux and the bulk components. In all cases it was found that the non-conservative nature of positronium (Ps) formation leads to all those processes. The recently developed positron trap at ANU in Canberra, Australia, has been used to cover the cross section data for a large number of gases, and our main interest has been focused on water vapour and other organic molecules that would provide the physical basis for modeling positron interaction with living organisms. Such data now exist, together with the corresponding transport data that show all the features induced by the Ps formation cross section. Finally, our improved ability to model collisions of positrons in gases has made it possible to model a gas-filled Surko trap and show how distributions of positrons develop from the initial beam like distributions and how positrons are thermalized. In collaboration with S.J. Buckman, J.Sullivan, C.Makochekanwa, A. Bankovic, S.Dujko, R. White, G. Malovic, M. Suvakov and S. Marjanovic [Preview Abstract] |
Wednesday, October 6, 2010 4:30PM - 5:00PM |
LW3.00002: Measuring accurate electron-molecule cross-sections for Plasma simulation Invited Speaker: An infrastructure has been built to measure absolute electron-atom (molecule) cross sections for elastic scattering, vibrational and electronic excitation and dissociative attachment. Emphasis is on low- energy capacity, entire range of scattering angles, low background and good resolution. Examples presented: \begin{itemize} \item[-]{CO: Elastic and vibrational excitation, differential and integral cross sections, in connection with simulations of planetary atmospheres of L Campbell and M Brunger.} \item[-]{HCN and C$_2$H$_2$: Absolute dissociative electron attachment cross sections, in connection with \textit{ab initio} 3-dimensional calculations of A E Orel and S T Chourou.} \item[-]{HCl and HBr: Absolute dissociative electron attachment cross sections, in connection with nonlocal resonance calculations of J Fedor, M Cizek, V McKoy and coworkers.} \item[-]{Kr: Absolute elastic and electronic excitation cross sections, in connection with $B$-spline $R$-matrix calculations of K Bartschat and O Zatsarinny.} \end{itemize} \\[4pt] In collaboration with Olivier May and Dusan Kubala, University of Fribourg. [Preview Abstract] |
Wednesday, October 6, 2010 5:00PM - 5:15PM |
LW3.00003: Electron Collision Cross Sections for ``Plasma-Like'' Gas Mixtures Leigh Hargreaves, Jessica Francis-Staite, Michael Brunger, Stephen Buckman Measurements of electron collision cross sections for a ``plasma-like'' gas mixture are presented. A gas target was prepared by thermally dissociating a CF$_{3}$I precursor to form a gas mixture containing species which are commonly observed in industrial plasmas. Products included stable molecules such as CF$_{3}$I, I$_{2}$ and C$_{2}$F$_{6}$, as well as neutral radicals such as CF$_{3}$ and I. Subsequently, the elastic differential cross sections for the gas mixture were measured for energies between 5 -- 50 eV. The measured data represents an important step towards the compilation of a sufficiently extensive database to meet the modeling needs of the low-temperature plasma industry. [Preview Abstract] |
Wednesday, October 6, 2010 5:15PM - 5:30PM |
LW3.00004: Elastic electron scattering from atomic and molecular iodine K. Bartschat, O. Zatsarinny, F. Blanco, G. Garcia We report calculations for elastic electron scattering from atomic and molecular iodine. An accurate knowledge of these cross sections is one of the key ingredients to indirectly infer the cross section for electron scattering from the CF$_3$ radical, after measuring the ``mixed beam'' cross section that results from the pyrolysis of a CF$_3$I parent molecule into species such as CF$_3$, I and I$_2$, whose relative abundance is known from a TOFMS. The calculations for e$-$I scattering were performed with the fully relativistic Dirac $B$-spline $R$-matrix (DBSR) method~[1]. We closely coupled the two fine-structure states $\rm (5s^25p^5)^2P_{3/2,1/2}$ and seven pseudo-states that were constructed to ensure the correct dipole polarizability of these states. The calculations for e$-$I$_2$ scattering were performed with a phenomenological model potential [2]. Their reliability was tested by comparing the predictions of the approach for the atomic target with the DBSR results.\\[4pt] [1] O.~Zatsarinny and K.~Bartschat, Phys.`Rev. A~{\bf 77} (2008) 062701.\\[0pt] [2] F.~Blanco and G.~Garc\'ia, Phys.~Rev. A~{\bf 67} (2003) 022701. [Preview Abstract] |
Wednesday, October 6, 2010 5:30PM - 5:45PM |
LW3.00005: Cross sections for electron scattering from the CF$_{3}$ radical Leigh Hargreaves, Jessica Francis-Staite, Michael Brunger, Stephen Buckman, Carl Winstead, Vince McKoy Electron interaction cross sections with fluorocarbon radicals, such as CF, CF$_{2}$ and CF$_{3}$, are a key data need for modeling industrial plasmas used to manufacture semiconductor devices. Here measurements of low-energy electron scattering cross sections for the CF$_{3}$ radical are presented. The gas target was prepared by thermal dissociation of a CF$_{3}$I precursor forming a gas mixture containing, amongst other products, CF$_{3}$. Time-of-flight analysis was used to characterize the gas mixture and, subsequently, the elastic differential cross sections for CF$_{3}$ were measured for energies between 5 -- 50 eV. The measured data are compared with new Schwinger Multichannel calculations. [Preview Abstract] |
Wednesday, October 6, 2010 5:45PM - 6:00PM |
LW3.00006: Electron attachement to CF$_2$ and preliminary results for C$_2$F$_5$ Thomas Field, Karola Graupner, Christopher Mayhew, Sean Haughey, Judith Langer, Eugen Illenberger Experiments have been performed to investigate electron attachment to highly reactive molecular fragments CF$_2$ and C$_2$F$_5$. CF$_2$ was generated in a plasma discharge of C$_3$F$_6$ (perfluoropropene) and by reaction of hydrogen atoms with CF$_3$I. The C$_2$F$_5$ radical was generated by reaction of C$_2$F$_5$I with hydrogen atoms. Electron attachment experiments were made with ERIC, which includes a low energy pulsed trochoidal electron gun and a small linear time-of-flight mass spectrometer (Field et al., J. Phys. B 38 (2005) 255). In the case of CF$_2$ no electron attachment was observed, despite theoretical prediction of a significant dissociative electron attachment cross section (Rozum et al., J. Phys. Chem. Ref. Data 35 (2006) 267). An upper limit for the dissociative electron attachment cross section of CF$_2$ has been estimated as 5$\times$10$^{-4}$ \AA$^2$ at 1.8 eV, which is the thermodynamic threshold for F$^-$ formation from CF$_2$. In the case of C$_2$F$_5$ the data are under analysis and results will be presented at the meeting. [Preview Abstract] |
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