Graduate Thesis Or Dissertation
 

Satellite structure in the gas-phase X-ray photoelectron spectra of planar unsaturated molecules

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  • The satellite structure ("shake-up") present in the X-ray photoelectron spectra of several planar molecules containing double bonds has been investigated in the gas phase. This set consists of three five-membered heterocyclic molecules, ethylene and two of its fluorinated analogs, and benzene with three of its fluorinated analogs. In addition, core ionization potentials have been accurately measured for the heterocyclic molecules. It is found that measuring satellites in the gas phase provides a clearer picture than do previous measurements done in the solid phase for the heterocyclic molecules; inelastic scattering structure, which is present in the same energy region as shake-up satellites can be easily minimized in the gas phase but not in the solid phase. Theoretical predictions of the relative intensities and energies of the satellites have been made within the sudden approximation using CNDO/2 wave functions. Agreement between theory and experiment is fair (within about a factor of two), suggesting that such wave functions are only of qualitative value. The distinct low-energy satellites present in the spectra have been assigned to transitions from occupied to unoccupied π orbitals. The unresolved high-energy satellites are, however, not so clearly understood. According to the theory, they are due in part to one-electron excitations in the valence a orbitals. However, certain high-energy satellites presumably result from single excitations to Rydberg-like or continuum orbitals ("shake-off") and multiple excitations. States such as these are not predicted by the theory because of the limited basis set. The wave functions are used to study charge rearrangement accompanying core ionization. It is found that in general, electrons are transferred from the atoms possessing the most electron density in the ground ionic state as the excited ionic states are populated. In certain cases, there is a direct relationship between the atomic character of the molecular orbitals involved in a given shake-up transition and the transition energies for core ionization at the different atoms. In many cases, the low-lying excited states of the ion are essentially those of the neutral molecule. However, if the molecular symmetry is modified by photoionization, other charge rearrangements that would not be expected from consideration of the neutral species can take place.
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