Abstract :
Continuum multiple scattering (CMS-X α) calculations are used to investigate methyl iodide photoionization dynamics. The validity of atomic versus molecular models of behaviour for the localised iodine 4d and 5p (lone pair) electrons, where conflicting experimental interpretations have been offered, is examined. A good account of all the available experimental data is obtained. While the calculations provide some limited support for an atomic-like description of the iodine 4d−1 phenomenological cross-section, the atomic picture has little validity for the 5p−1 ionization. Variations in both cross-section and β parameter which were previously ascribed to the Cooper minimum are identified to be more likely a core-valence shell interaction, opening above the 4d−1 threshold. The other valence shell channels are similarly affected. Significant l-wave mixing, induced by the molecular anisotropy, is found to result in the creation of mixed parity final states, contrary to the atomic model. In these circumstances odd and even harmonic components can interfere to create orientation (odd harmonic terms) in the photoelectron angular distribution (PAD). Molecule frame PADs are calculated and compared to experimental data for the ionization of oriented molecules, confirming an intuitive expectation of asymmetric electron emission from an asymmetric molecule. The CH3 group therefore acts as a non-negligible perturbation, even on quite localised iodine 5p electrons, and the molecular environment is a paramount factor.
