Bulletin of the American Physical Society
60th Gaseous Electronics Conference
Volume 52, Number 9
Tuesday–Friday, October 2–5, 2007; Arlington, Virginia
Session RR2: Electron-Molecule Collisions |
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Chair: Rich Fernsler, Naval Research Laboratory Room: Doubletree Crystal City Crystal Ballroom B |
Thursday, October 4, 2007 1:30PM - 2:00PM |
RR2.00001: Electron-Driven Ionization of Processing Gases: Status and Perspectives Invited Speaker: In 1985, experimentally determined absolute partial and total electron-impact ionization cross sections for 31 molecules had been reported in the literature [1]. Today, the number of molecules, for which cross sections have been measured, exceeds 100. Experimental ion formation studies have included work involving free radicals and clusters as targets as well as the study of metastable ionic decay routes. Much effort has been devoted to the study of electron-driven ionization of molecules and free radicals of importance top the plasma processing community. These include many halogen-bearing species, but also molecules such as diborane and silane. While a rigorous, fully quantum mechanical theoretical treatment of molecular ionization processes is still impossible (because of the complexity of the ionization process and the complexity of the targets under study), semi-rigorous approaches such as the method of Khare and co-workers, the Binary Encounter Bethe (BEB) approach of Kim and co-workers, and the Deutsch-M\"{a}rk (DM) formalism (see Ref. [2] for details of these theoretical approaches) have made [2] significant progress. This talk will review recent progress in the experimental (and to a lesser extent theoretical) progress in the field of electron-induced ionization of processing gases. Special emphasis will be placed on recent studies of the electron-impact ionization of Cl-bearing molecules and radicals and the respective role direct vs. indirect ionization processes for these targets. This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, US Department of Energy. \newline \newline [1] ``Electron Impact ionization'', T.D. M\"{a}rk, G.H. Dunn (editors), Springer Verlag: Vienna (1985) \newline [2] H. Deutsch, K. Becker, S. Matt, and T.D. M\"{a}rk, Int. J. Mass Spectrom. \underline {\textit{197}}, 37 (2000) [Preview Abstract] |
Thursday, October 4, 2007 2:00PM - 2:30PM |
RR2.00002: Low Energy Elastic Scattering of Electrons from Polyatomic Molecules Invited Speaker: Measurements and calculations of the elastic scattering of electrons from ``large'' polyatomic molecules, e.g. alcohols, CH$_{3}$OH and C$_{2}$H$_{5}$OH are presented. These measurements are made possible by using a modified form of the relative flow method which uses a thin aperture source of target gas, instead of the conventional tube sources used in the past. The aperture source provides an angular distribution of gas which is independent of the pressure behind the source, provided the gas mean-free path does not exceed the aperture thickness. This property has been tested using C$_{2}$H$_{2}$, N$_{2}$ and He [1]. The experimental data were taken at incident energies of 2eV, 5eV, 10eV, 15eV, 20eV, 30eV, 50eV and 100eV. The theory uses the variational multi-channel Schwinger method with polarization effects [2], and very good agreement between experiment and theory is observed in general. The talk will focus on the experimental implementation of the modified relative flow method and its validity. \newline \newline [1] M. A. Khakoo, K. Keane, C. Campbell, N. Guzman and K. Hazlett, \textit{Low Energy Elastic Electron Scattering from Ethylene}, submitted to J. Phys. B. June-2007. \newline [2] C. Winstead and V. McKoy, Adv. At. Mol. Opt. Phys. \textbf{36}, 183 (1996). [Preview Abstract] |
Thursday, October 4, 2007 2:30PM - 3:00PM |
RR2.00003: Electron Scattering from Molecules Invited Speaker: Cross sections for low-energy electron-molecule scattering compose a relevant data basis for modeling discharge environments and for understanding important astrophysical and biological phenomena. In this talk I will present our progress with the Schwinger multichannel (SMC) method with pseudopotential (M. H. F. Bettega, L. G. Ferreira, and M. A. P. Lima, Phys. Rev. A {\bf 47}, 1111 (1993)) recently implemented with a better description of the target (R. F. da Costa, F. J. da Paix\~ao and M. A. P. Lima, \emph{J. Phys. B} {\bf 37}, L129 (2004)) to study electronically inelastic processes in electron-molecule scattering. Motivated by the relative success of our applications for H$_2$ (R. F. da Costa, F. J. da Paix\~ao and M. A. P. Lima, \emph{J. Phys. B} {\bf 38}, 4363 (2005)) and N$_2$ (R. F. da Costa and M. A. P. Lima, \emph{Int. J. Quantum Chem.} {\bf 106}, 2664 (2006); {\bf ibid}, \emph{Phys. Rev. A} {\bf 75}, 022705 (2007)) molecules, we have obtained inelastic cross sections for few electronic states of CO, C$_2$H$_4$, C$_4$H$_4$O, and two important alcohols, CH$_3$OH and C$_2$H$_5$OH. For e-furan scattering processes (M. H. F. Bettega and M. A. P. Lima, J. Chem. Phys. {\bf 126}, 194317 (2007)) I will discuss the influence of low-lying electronic states on the elastic cross sections and more important, the influence of polarization effects on the electronic excitation of these states by electron impact. For ethanol and methanol molecules I will show a comparative study for elastic and electronic inelastic processes that is being carried out within an international effort involving three Brazilian and two American groups. [Preview Abstract] |
Thursday, October 4, 2007 3:00PM - 3:15PM |
RR2.00004: Electron cooling rates in the atmospheres of Mars and Venus Laurence Campbell, Michael Brunger, Thomas Rescigno Vibrational excitation of molecules by electron impact, followed by radiative decay, is a cooling mechanism in planetary atmospheres. As carbon dioxide is the dominant constituent below 200 and 140 km in the atmospheres of Mars and Venus respectively, the electron cooling rates for CO$_2$ are required in modeling the atmospheres of these planets. Such cooling rates were determined many years ago, but new measurements and calculations of the electron impact cross sections have since become available. There have also been new measurements of the atmospheric parameters, such as of the electron density on Mars, that are required in the calculations. Therefore we have assembled a new data base of electron impact cross sections for CO$_2$, based on the more recent measurements and calculations, and used it to calculate electron cooling rates for Mars and Venus. One result is that we predict a larger cooling rate for the altitude range 100--150 km in the Martian atmosphere. This may explain why temperatures observed in this region are less than predicted. [Preview Abstract] |
Thursday, October 4, 2007 3:15PM - 3:30PM |
RR2.00005: Absolute elastic cross sections for intermediate energy electron scattering from CF$_2$ radicals Leigh Hargreaves, Jessica Francis-Staite, Todd Maddern, Michael Brunger, Stephen Buckman We report on differential and integral elastic cross sections for electron scattering from CF$_2$ radicals. The energy range of the present measurements is 20-50 eV, while the angular range of the differential cross sections is 20-135$^{\circ}$. A full description of the new apparatus, and the techniques that were used to make these measurements, will be given at the meeting. [Preview Abstract] |
Thursday, October 4, 2007 3:30PM - 3:45PM |
RR2.00006: Total and total ionization cross sections for NH$_{x}$ and NF$_{x}$ (x = 1,2 and 3) on electron impact C.G. Limbachiya, M. Vinodkumar, S. Gangopadhyay, K.N. Joshipura Electron-impact induced ionization cross-sections for NH$_{3}$ and NF$_{3}$ and their chemically reactive free radicals, NH$_{X}$ {\&} NF$_{X}$ (x = 1,2) are relevant to processes in planetary {\&} cometary atmospheres. In this paper we have reported the total (complete) and total ionization cross sections for molecules NH$_{X}$ and NF$_{X}$ (x = 1,2 and 3) on electron impact from threshold to 5 keV. We used complex optical potential formalism (SCOP) [1] to calculate the total inelastic cross sections Q$_{inel}$ and total (complete) cross sections Q$_{T}$ Q$_{inel}$ includes Q$_{ion}$ and we have developed a semi-empirical method, Complex Scattering Potential -- ionization contribution (CSP-ic) to extract ionization cross sections Q$_{ion}$ from calculated Q$_{inel}$ [2]. \textbf{References} [1] M.Vinodkumar, K.N.Joshipura, C.G.Limbachiya {\&} B.K.Antony, Eur. J. Phys. D. \textbf{37} (2006) 67 [2] M.Vinodkumar, K.N.Joshipura, C.G.Limbachiya {\&} B.K.Antony, Phys. Rev A \textbf{74 }(2006) 022721 [Preview Abstract] |
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