The Exchange Excitation of Helium and the Hydrogen Molecule by Low Energy Electrons

Citation

Cartwright, David Chapman (1968) The Exchange Excitation of Helium and the Hydrogen Molecule by Low Energy Electrons. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/51DM-6585. https://resolver.caltech.edu/CaltechETD:etd-09112002-165521

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The theory of rearrangement collisions involving composite particles is reviewed and the "post", "prior" discrepancy discussed. Two recent improvements to the Born-Oppenheimer approximation, the Ochkur (O), and Ochkur-Rudge (OR) approximations are reviewed. These approximations are then used to calculate the total cross section for the electronic excitation, of molecular hydrogen by low energy electrons. Excitations from the ground [...] state to the first [...] and second [...] triplet states are treated. All nuclear motions are taken into account. It is found that the first triplet cross section is sensitive to the choice of the ground state wave function whereas the second is not. The former is also sensitive to the quality of excited state wave function used. The results using the (O) approximation are significantly larger than those of the (OR) approximation, and the maximum cross section occurs at a somewhat lower energy. Use of the separated atom approximation produces results significantly lower than those obtained by including all the multicenter terms in the scattering amplitude. The calculations are carried out using the zeta function expansion method and are quite lengthy. The sum of the first and second (OR) triplet cross sections agrees well with a recent approximate experimental determination of the cross section for electron impact dissociation of [...] into 2H. These approximations are also used to calculate the total cross sections for excitation of helium from the ground to the [...] and [...] states. These cross sections are sensitive to the ground and excited state wave functions used. The (OR) results agree well with the available experimental data for these excitations. The differential cross sections for these excitations are also calculated. Since there is no experimental data for the [...] processes or for the [...] excitation in He, the quality of these calculations cannot be evaluated. The calculated angular distributions for the [...] excitation in He agree well with one set of experimental data but disagree with the other. The (OR) approximation is found to give quite good results for total cross sections but its value in predicting angular distributions cannot be evaluated until more experimental data are available.

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