The Angular Resolved Photoelectron Spectroscopy of Various Polyatomic Molecular Systems

This thesis describes the study of the angular resolved photoelectron spectroscopy of a series of polyatomic molecules. The spectrometer consists of a He I radiation source, a scattering chamber, and a rotatable detection system which includes a set of electrostatic lenses, a hemispherical electrostatic kinetic energy analyzer and an electron multiplier. Angular distributions are determined from the variation in intensity as the detection system is rotated about the center of the scattering chamber. The theory of photoionization is discussed semiclassically, as the interaction of an atom or molecule treated quantum mechanically with a classical radiation field. Some recent calculations of the asymmetry parameter for valence electrons are briefly reviewed. Photoelectron angular distributions were measured for acetylene, propyne, 1-butyne, and 2-butyne. The asymmetry parameters of propyne, 1-butyne, and 2-butyne have been determined for the first time. Trends in the asymmetry parameter, ionization potentials and band shapes were studied. It was determined that the parity favoredness rules of Chang failed to account for the behavior of the asymmetry parameter of the molecule despite the symmetry of the molecule. Instead, acetylene and its alkylated analogs follow the trends in β observed in studies of the methylated ethenes. Additionally, the semi-empirical rule that the β values of π orbitals are higher than for σ orbitals was violated in this series. Acetylene and propyne possess σ orbital with β values significantly higher than the π orbitals. Two principal substituent effects were observed: 1) a systematic decrease in the first ionization potential and 2) a similar decrease in the asymmetry parameter of the X̃ band with increasing alkylation. The photoelectron angular distributions were measured for formaldehyde, acetaldehyde, and acetone. The asymmetry parameter has been determined for the first time for acetaldehyde, and, with the exception of the first band, for acetone. This study has shown that the beta values of the X̃ nO bands of these molecules are, within experimental error, invariant with respect to methyl substitution, results that are consistent with the nonbonding characteristics of the molecular orbitals. The à πC = O bands, however, show a strong decrease in the asymmetry parameter of approximately 0.2 per methylation in a manner similar to that observed previously in the methylated ethenes and ethynes. The expected systematic decrease in first ionization potential with substitution was also observed. Lastly, HAM/3 calculations were performed to determine the ionization potentials of some substituted carbonyls and to examine the excitation energies of ethylene and its methyl and fluoro derivatives to evaluate the method's usefulness to studies in electron impact spectroscopy. There was generally good agreement between the ionization potential calculated by this method and experimentally determined values. Agreement between the calculated values of the excitation energies and the experimental were reasonable but the method was not sensitive enough to reproduce the trends observed with increasing substitution of the chromophore.