Bulletin of the American Physical Society
2005 58th Gaseous Electronics Conference
Sunday–Thursday, October 16–20, 2005; San Jose, California
Session CM1: Dissociation, Recombination, and Attachment |
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Chair: Stephen Buckman, Australian National University Room: Doubletree Hotel Pine |
Monday, October 17, 2005 10:00AM - 10:30AM |
CM1.00001: Theoretical Studies of Electron-Molecule Collision Processes Invited Speaker: Electron collisions with molecules and molecular ions that lead to excitation and dissociation play a key role in a number of environments, since they produce the radicals and molecular fragments that initiate and drive the relevant chemistries. Examples range from the technologically important plasmas used in plasma enhanced chemical vapor deposition, to planetary atmospheres and interstellar clouds, to DNA damage driven by secondary electron cascades produced by radiation. In general, due to the large mass difference between the electron and target, the cross section is dominated by resonant processes, where the electron can temporarily attach to the molecule and change the forces felt between its atoms for a period of time comparable to a vibrational period. This can lead to resonant vibrational excitation and dissociative attachment, for neutral targets, or dissociative recombination in the case of ions. In this talk, I will outline the basic theory that underlies these processes, and our approach to study them. I will illustrate these methods with application to dissociative attachment in ClCN and BrCN, vibrational excitation and dissociative attachment in NO and CF, and dissociative recombination in the HCN$^{+}$ and HNC$^{+}$ systems. This work was supported by DOE-OBES Division of Chemical Sciences and NSF PHY-02-44911. [Preview Abstract] |
Monday, October 17, 2005 10:30AM - 11:00AM |
CM1.00002: Recent Studies of Cross Sections, Rates and Branching Ratios for the Recombination of Ions in Industrial Plasmas Invited Speaker: Branching ratios for the dissociative recombination of hydrocarbon ions with the general formula C$_{2}$H$_{3}^{+}$, C$_{3}$H$_{m}^{+}$ (m=1- 8) and C$_{4}$H$_{m}^{+}$ (m=1-9) have been measured using the ASTRID storage ring in Aarhus [1-3]. The detector used did not have sufficient energy resolution to distinguish hydrogen atoms either free or attached to carbon atoms so the relative distributions of carbon atoms among the dissociation products were measured. For most of the ions that were in linear isomeric form, the fragmentation patterns were predictable from the structure of the parent ion. For cyclic isomers however, this was not so clear and indications are that ring opening occurs prior to dissociation. Results obtained for C$_{2}$H$_{3}^{+}$ and C$_{3}$H$_{7}^{+}$ are in excellent agreement with studies performed by the CRYRING group in Stockholm [4,5]. Rare gas hydride ions are important in several industrial applications and recent cross section measurements have shown that while NeH$^{+} $ [6] does recombine at low energies, ArH$^{+}$ [7] does not. Both these ions display recombination resonances and dissociative excitation onsets at higher electron energies. The fluorocarbon ions, CF$^{+}$, CF$_{2}^{+}$ and CF$_{3}^{+}$ that are found in semiconductor etching plasmas have been examined in a collaborative effort with the CRYRING group and results for cross sections and branching ratios will be presented [8-10]. Work supported by the European Union, AFOSR, EOARD, the Danish and Swedish Research Councils and the French Centre National pour la Recherche Scientifique (CNRS) \\ \\ (1) J.B.A. Mitchell, et al. Int. J. Mass Spec. 227, 273, 2003 \\ (2) G. Angelova et al (2004) Int. J. Mass Spec. 232, 195, 2004 \\ (3) G. Angelova, et al Int. J. Mass Spec. 235,7, 2004. \\ (4) S.S. Khalori, et al Astron {\&} Astrophys. 391, 1159, 2002 \\ (5) A. Ehlerding, et al J. Phys. Chem. A 107, 2179, 2003 \\ (6) J.B.A. Mitchell et al. J. Phys. B 38, 693, 2005 \\ (7) J.B.A. Mitchell et al. J. Phys. B 38, L175, 2005 \\ (8) G. Angelova et al. J. Phys. B 37, 1, 2004 \\ (9) O. Novotny et al. J. Phys. B 38, 1471, 2005 \\ (10) A.Ehlerding et al. (In preparation) [Preview Abstract] |
Monday, October 17, 2005 11:00AM - 11:15AM |
CM1.00003: Dissociative Recombination of the Rare Gases J. Royal, V. Ngassam, A.E. Orel We report cross sections for the dissociative recombination of the He$_{2}^{+}$,Ne$_{2}^{+}$ and Ar$_{2}^{+}$ molecular ions following collisions with low energy electrons. Dissociative recombination of the rare gases are important processes in the ionosphere as well as laboratory plasmas and gaseous discharges. This research will explore the similarities and differences between the three processes as well as the trend in the DR cross section as the atomic mass increases. The energy positions and autoionization widths of these doubly excited dissociative states are obtained from electron scattering calculations using the Complex Kohn Variational method. The dissociation dynamics is studied using either multichannel quantum defect theory (MQDT), discrete variable representation (DVR) with exterior complex scaling (ECS), or time-dependent wave packet methods. The calculated cross sections will be compared to available experiment. Work supported by the NSF PHY-02-44911 and The Center for Biophotonics, an NSF Science and Technology Center PHY 0120999, and NATO science program PST.GLG.9794033. [Preview Abstract] |
Monday, October 17, 2005 11:15AM - 11:30AM |
CM1.00004: Dissociative Electron Attachment to H$_2$O and H$_2$S Daniel Haxton, Thomas Rescigno, C. William McCurdy Dissociative electron attachment (DA) to H$_2$O is of direct importance for both biological and techological systems. The calculations on H$_2$O and H$_2$S presented comprise the first \textit{ab initio} treatment of DA to a polyatomic molecule employing the full dimensionality of nuclear motion. Cross sections obtained for DA via the $^2$B$_1$ state of H$_2$O agree well with experiment, reproducing the high degree of vibrational excitation of the OH fragment. Several interesting features of the $A'$ manifold of resonances for H$_2$O have been discovered, including a conical intersection between the $^2$A$_1$ and $^2$B$_2$ Feshbach resonances and a branch-point degeneracy between the $^2$B$_2$ shape and Feshbach resonances. This latter feature has no analogue in bound-state theory. We show results of recent calculations on the Renner-Teller coupled $^2$A$_1$ and $^2$B$_1$ surfaces, and on electronically coupled diabatic $^2$A$_1$ and $^2$B$_2$ surfaces. The angular dependence of the H$^-$ + OH channel for the $^2$B$_1$ state of H$_2$O and that of the analagous channel and state of H$_2$S have been calculated by incorporating the mixing of different partial waves into the entrance amplitude, and for H$_2$S, the variation of this mixing with geometry. [Preview Abstract] |
Monday, October 17, 2005 11:30AM - 11:45AM |
CM1.00005: Dissociative recombination of HCO$^+$ Stefano Tonzani, Aasa Larson, Viatcheslav Kokoouline, Ivan Mikhaylov, Robin Santra, Chris H. Greene The dissociative recombination (DR) mechanism of the important interstellar ion HCO+ at low electron energies is studied theoretically. Our work suggests that DR is driven through capture into Rydberg states, and that no direct mechanism operates at collision energies below a few eV. Our approach includes accurate quantum chemical calculations, the three-dimensional vibrational dynamics and a treatment of the joint electron-nuclear dynamics is treated using quantum defect theory and a frame transformation approach. Results are presented for the calculated autoionization widths of Rydberg states and the dissociative recombination cross sections, and compared with available experiments. [Preview Abstract] |
Monday, October 17, 2005 11:45AM - 12:00PM |
CM1.00006: Numerically Solvable Model of Low-Energy Resonant Electron-Molecule Collisions without Born-Oppenheimer Approximation K. Houfek, C.W. McCurdy, T.N. Rescigno We suggest a simple model with one nuclear and one electronic degree of freedom that can be solved exactly (without the Born-Oppenheimer approximation) employing the exterior complex scaling method and the finite-element method with discrete variable representation. Using this model it is possible to study basic electron-molecule collisions: the vibrational excitation of a molecule by electron impact and the dissociative electron attachment to a molecule. The full Hamiltonian of our model is $H = -1/2\mu\,d^2/dR^2 - 1/2\,d^2/dr^2 + V_0(R) - \lambda(R) e^{-\alpha r^2} + l(l+1)/2r^2$ where $V_0(R)$ is a Morse potential for the vibrational motion of the neutral molecule, the forth term describes the interaction between the molecule and the electron and the centrifugal term with $l \not= 0$ is added to provide a resonant behavior in our system. The suggested model enables us to compare various approximations used in low-energy resonant electron-molecule collisions (for example the boomerang model or the nonlocal resonance model) with the exact solution and to investigate when these approximations are valid. [Preview Abstract] |
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