Angular Distributions in Photoelectron Spectroscopy

Author: Sell, Jeffrey Alan

Year: 1979

Degree: Dissertation (Ph.D.)

Advisor: Kuppermann, Aron

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/3r2z-er47

Abstract

This thesis describes the measurements of the angular distributions in the photoelectron spectroscopy of diatomic and polyatomic molecules. The apparatus used was constructed previously, and basically consists of a He! radiation source, a scattering chamber, a set of electrostatic lenses, a 180° hemispherical electrostatic kinetic energy analyzer, and detector. The lenses, analyzer and detector are mounted on a gear and rotate about the center of the scattering chamber allowing the measurements of the photoelectron angular distributions.

The theoretical section outlines the quantum mechanics of the photoionization process and briefly describes recent attempts to calculate the photoelectron asymmetry parameter β for atoms and molecules. The results for the hydrogen molecule are compared to the experimental values measured by the present author and previous ones, with fairly good agreement.

Measurements of β were carefully performed for 24 vibrational transitions of carbon monoxide using both 537 Å and 584 Å radiation as excitation sources. Its variation with final state vibrational quantum number was found to be quite pronounced and non-monotonic. By consideration of this and other experimental and theoretical data, it was concluded that an autoionizing state of CO should exist at 584 Å, and that β is very sensitive to the presence of such a state.

The photoelectron angular distributions were measured for benzene, fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene. Trends in the asymmetry parameter β as well as in the spectral band positions and shapes across this series of molecules led to the assignments of all of the π orbitals and some of the σ orbitals associated with the features in the spectra.

Angular distributions of the photoelectrons ejected from SF6 were measured using Hel radiation. By considering the trends in the β parameter across the spectral bands, as well as other data, the assignments of the corresponding orbitals to these bands were made.

Measurements of the photoelectron angular distributions in the heterocyclic molecules furan, thiophene, and pyrrole were performed. The trends in β and in the shapes and positions of the bands in the spectra were used to assign the peaks that correspond to ionization from the π orbitals in these molecules.

The photoelectron angular distributions of the lowest IP carbon-carbon π electrons in ethylene and all of its fluorinated derivatives, and benzene and some of its fluorinated derivatives were measured. It was noticed that the value of β for the maximum intensity of the lowest IP π band decreased monotonically in the fluoroethylene series from a value of 1.25 for ethylene to 0.05 for perfluoroethylene. On the other hand it decreased non-monotonically in the fluorobenzene series from a value of 1.23 in benzene to 0.65 for perfluorobenzene. This difference in the behavior of β upon fluorine substitution in the two series of molecules was attributed to the difference in the interaction of the fluorine π lone pair electrons with the carbon-carbon 7T electrons (the resonance effect).

The photoelectron angular distributions were measured for both σ and π electrons in all of the fluorine derivatives of ethylene. High resolution (20 meV) spectra were reported for the bands which have vibrational structure. Trends in the ionization potentials, the variation of β across bands, and the vibrational frequencies were used to assign several orbitals to the corresponding photoelectron bands.

Finally, all of the above results are briefly summarized in terms of the relationship of β to various characteristics of the molecular orbitals. In addition, future work in this field is suggested.

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