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UBC Theses and Dissertations

Studies of molecular orbitals by electron momentum spectroscopy Clark, S. A.


The binding energies and momentum distributions of all of the valence orbitals of (CH₃)₂O, PH₃, CH₄ and SiH₄ have been measured by high momentum resolution electron momentum spectroscopy. The binding energy spectra have been compared to Green's function and configuration interaction calculations from the literature and with new calculations performed in collaboration with co-workers at Indiana University and Universitat Braunschweig. For PH₃, CH₄ and SiH₄, near Hartree-Fock limit calculations of the momentum distributions and very accurate calculations of the ion-neutral overlap using MRSD-CI wavefunctions to describe the ion and target have been performed in collaboration with co-workers at Indiana University. Good agreement is obtained between the (CH₃)₂O measurements and the momentum distributions calculated from relatively simple wavefunctions, except in the case of the outermost orbital. The effects of diffuse and polarization functions in the basis sets, and also the influence of molecular geometry, have been investigated. Comparison of the momentum distributions of the outermost orbitals of H₂O, CH₃OH and (CH₃)₂O demonstrates a delocalization of charge density with methyl substitution. The measured momentum distributions of PH₃, CH₄ and SiH₄ are compared with near Hartree-Fock limit calculations as well as ion-neutral overlap calculations in which the ion and neutral wavefunctions are described by multireference, singly and doubly excited, configuration interaction calculations. In each case, the experimental results are well modelled by the near Hartree-Fock limit calculations, and there is little difference between the Hartree-Fock limit and ion-neutral overlap calculations. A significant splitting of the 4a₁ (inner valence) pole strength is observed for PH₃, but the inner valence strength is largely contained in the main peak for both CH₄ and SiH₄. Green's function calculations quantitatively reproduce these results. Ion-neutral overlap calculations using MRSD-CI wavefunctions to describe the ion and target have been performed for HF, HCl, Ne and Ar. These are compared with previously published EMS measurements of the momentum distributions. Very poor agreement between theory and experiment is obtained for HF and HCl. The theoretical and experimental results for all of the hydrides CH₄-HF and SiH₄-HCl as well as Ne and Ar are reviewed.

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