Dynamics of Electron-Molecule Collisions

Citation

Lima, Marco Aurelio Pinheiro (1986) Dynamics of Electron-Molecule Collisions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/T10Z-7D32. https://resolver.caltech.edu/CaltechTHESIS:11042019-165338290

Abstract

We have developed a multichannel formulation for low-energy electron-molecule collisions based on the Schwinger variational method. An important feature of this formulation is that it is capable of dealing with some important aspects of electron-molecule collisions such as electronically inelastic scattering and nonlinear targets. The formulation also allows for the inclusion of a substantial number of closed electronic channels to represent polarization effects, which are very important at low-impact energies.

To assess the accuracy with which polarization is represented in this formulation we have calculated elastic integral and differential cross sections for e — H₂ collisions. We have obtained very good agreement between our results and available theoretical and experimental data. We have also examined the shape resonances in e — CO and e — N₂ collisions, where polarization plays a very important role in determining the resonance parameters.

Our first application to electron-nonlinear molecule collisions was for e — CH₄ scattering. At the static-exchange level of approximation our differential cross sections are in very good agreement with existing measurements at about 7 eV and higher energies. For incident energies below this an interesting feature of the small angle differential cross sections is seen only when polarization effects are included in the calculation. We have also obtained elastic differential and momentum transfer cross sections for e — H₂O scattering. Our calculated differential cross sections agree well with available experimental data, which extend only to 120°. The differential cross sections, particularly at 15 and 20 eV, show significant backward peaking. This peaking occurs in the experimentally inaccessible region beyond scattering angle of 120°, and, as we will see, it has an important implication in the determination of momentum transfer cross sections.

We also obtained differential and integral cross sections for the electron impact excitation of the b³Σ⁺_u, a³Σ⁺_g, c³Π_u and B¹Σ⁺_u states of H₂ at the two state approximation. In contrast to the triplet excitations, the B¹Σ⁺_u is a dipole-allowed transition, a feature which introduces several new interesting aspects. Our results for the excitation of the b³Σ⁺_u and B¹Σ⁺_u states are in good agreement with experimental data at all energies.

The results we obtained with the Schwinger multichannel method are very encouraging and illustrate the potential utility of these calculated cross sections. The results of such studies can clearly complement experimental efforts to determine absolute values of these electron-molecule scattering cross sections.

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