Date of Award

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

A. Ravi. P. Rau

Abstract

The coupling of electronic and nuclear degrees of freedom in electron scattering from a homonuclear diatomic molecule has been studied using the frame transformation method in conjunction with the multichannel quantum defect theory (MQDT). Exploration of non-adiabatic effects resulting from large electron-molecule distances, resonances, and dissociation shows that the interconversion of energy between electronic and nuclear motion takes place mainly in the vicinity of the molecular target where the outermost electron couples strongly to the nuclei. MQDT enables us to describe these effects based on a body-frame quantum defect function, which can be obtained either from a short-range ab initio calculation (using Born-Oppenheimer approximation) or by fitting to the experimental data. This function thus provides sufficient information to characterize wavefunctions in asymptotic regions where the electron-molecule system may decay into different fragments (either the electron leaves the target, or else the two nuclei dissociate). The frame transformation method serves as a vehicle to convert the body-frame scattering parameter into laboratory-frame observables. We discuss two types of frame transformation methods for describing rovibrational coupling in this work. An energy-independent frame transformation is shown to be accurate enough to describe processes when the collision time of the scattering electron is negligible compared to the typical time for nuclear motion (i.e., when the energy dependence of the quantum defect is weak). In order to describe resonances (where the electron can be temporarily captured by the molecular target), we formulate an energy-dependent frame transformation which explicitly includes the energy dependence in the frame transformation without invoking any electron-molecule compound states. Finally, we make a preliminary study of processes involving competition between ionization and dissociation. Short-range rovibrational coupling is shown to govern such processes as well, for one class of molecular photoabsorption processes.

Pages

144

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