Bulletin of the American Physical Society
62nd Annual Gaseous Electronics Conference
Volume 54, Number 12
Tuesday–Friday, October 20–23, 2009; Saratoga Springs, New York
Session WF2: Electron Molecule Collisions |
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Chair: Don Madison, Missouri University of Science and Technology Room: Saratoga Hilton Ballroom 2 |
Friday, October 23, 2009 8:00AM - 8:30AM |
WF2.00001: Spin-Dependent Effects in Electron-Molecule Scattering Invited Speaker: This talk will review the study of electron-molecule collisions that use electron polarization as a probe of the collision dynamics. Compared with atoms, early experiments with molecular targets seemed to indicate a ``quenching'' of spin-dependent effects. Measurements by the Rice [1] and Muenster [2] groups showed that exchange cross sections for electron-molecule scattering were much smaller than those for atoms. These discrepancies were never adequately explained. More recently, measurements by our group and at Muenster have given similar results when collision-induced fluorescence is observed. We will discuss these latter fluorescence measurements with H$_{2}$ and N$_{2}$ targets in detail, and show how rotational resolution of the electron-impact excited states can resolve, at least partially, the ``quenching'' puzzle. \\[4pt] [1] G.H. Rutherford et alii, Rev. Sci. Instrum. \textbf{61}, 1460 (1990). \\[0pt] [2] T. Hegemann \textit{et alii}, J.Phys.B \textbf{26}, 4607 (1993). [Preview Abstract] |
Friday, October 23, 2009 8:30AM - 9:00AM |
WF2.00002: Electron-driven excitation and dissociation of molecules Invited Speaker: Due to the large difference in mass between the electron and the nuclei, when an electron collides with a molecule or molecular ion, there is inefficient transfer of energy from the electron into the motion of the nuclei, leading to little vibrational excitation or dissociation. However, in certain special cases, 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. Studies of dissociative recombination and attachment in several polyatomic systems have shown that simple one-dimensional models can fail to capture the correct dissociation dynamics. In our treatment of these processes we first carry out {\it ab initio} electron scattering calculations at fixed internuclear geometries to determine the resonant energy surfaces and the corresponding surface of autoionization widths using the Complex Kohn variational method. These resonance positions and widths are then used as input to a dynamics study to determine the cross-section and product distributions for the dissociation or excitation process. We will present results on a number of systems, including HCCH, HCN/HNC and HCCCN as examples of dissociative attachment and N$_2$H$^+$ and H$_2$O$^+$ for dissociative recombination. [Preview Abstract] |
Friday, October 23, 2009 9:00AM - 9:15AM |
WF2.00003: Electron attachment to halomethanes at high temperatures T.M. Miller, J.F. Friedman, L.C. Schaffer, A.A. Viggiano We have modified our high-temperature flowing-afterglow apparatus to include a movable Langmuir probe, a 4-needle reactant gas inlet, and a microwave discharge plasma source for the purpose of measuring electron attachment rate constants at high temperatures. We have focused initially on molecules which have very small attachment rate constants, k$_{a}$, at room temperature to see if their behavior at high temperatures can be described in Arrhenius fashion. We have reported k$_{a}$ for CH$_{3}$Cl, but only above 600 K, because the value at 600 K was quite small: 5.8 $\times $10$^{-12}$ cm$^{3}$ s$^{-1}$. The Arrhenius plot for these data imply k$_{a}$ = 10$^{-17}$ cm$^{3}$ s$^{-1}$ at 300 K, a value that is so small as to be immeasurable with any current apparatus. We now have k$_{a}$ for other halomethanes, CF$_{3}$Cl, CF$_{2}$Cl$_{2}$, and CH$_{2}$Cl$_{2}$. The halomethane data cover seven orders-of-magnitude in k$_{a}$. Electron attachment to CF$_{3}$Cl is endothermic by 143 meV at 300 K, but our measurements indicate that there is a barrier of about 400 meV, probably related to the energy at which the anion surface crosses that of the neutral. The reactions for CH$_{3}$Cl, CF$_{2}$Cl$_{2}$, and CH$_{2}$Cl$_{2}$ are exothermic, but our data again indicate large barriers to attachment which accounts for the extremely slow attachment at 300 K. From these data and literature measurements at 300 K, one can make educated guesses as to the behavior of k$_{a}$ for other halomethanes. [Preview Abstract] |
Friday, October 23, 2009 9:15AM - 9:30AM |
WF2.00004: Dissociative Electron Attachment to Polyatomic Systems S.T. Chourou, A.E. Orel We have performed a multi-dimensional computational treatment of the dissociative electron attachment (DEA) dynamics of 3 polyatomic systems; HCCH, HCN/HNC and HCCCCN to investigate predicted inherent polyatomic effects. We have considered the following reaction channels: C$_2$H$_2$ (X$^1\Sigma_{g}^+$, $\nu$)+ e$^-$($E$) $\rightarrow$ (C$_2$H$_{2}$)$^{-*}$ ($^2\Pi_g$) $\rightarrow$ C$_2$H$^-$($^1\Sigma^+$,$\nu'$) + H($^2S$), HCN/HNC ($X^1\Sigma^+, \nu$) + e$^-$($E$) $\rightarrow$ (HCN/HNC)$^{-*}$ ($^2\Pi_g$) $\rightarrow$ CN$^-$($^1\Sigma, \nu'$) + H ($^2S$) and HCCCN (X$^1\Sigma^+$, $\nu$)+ e$^-$($E$) $\rightarrow$ HCCCN$^{-*}$ $\rightarrow$ $\left\{ \begin{array}{ll} $CCCN$^-(^2\Sigma^+ , \nu'^-_I) + $H$(^2S)$: (I)$ ; \\ $CN$^-(^1\Sigma^+ , \nu'^-_{II}) + $HCC$(^2\Sigma^+ , \nu'_{II})$: (II)$ ; \\ $HCC$^-(^1\Sigma^+ , \nu'^-_{III}) + $CN$ (^2\Sigma^+ , \nu'_{III})$: (III)$ ; \\ $CC$^-(^2\Sigma^+ , \nu'^-_{IV}) + $HCN$ (^1\Sigma^+ , \nu'_{IV}) $: (IV)$ ; \end{array} \right.$. We carried out electron scattering calculations using the Complex Kohn Variational Method with respect to a suitable internal coordinate system to obtain the complex resonant energy surfaces. We use this as input to a dynamics calculation using the Multiconfiguration Time-Dependant Hartree approach. We then compare our DEA cross sections and branching ratios to available experimental results. [Preview Abstract] |
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