A THEORETICAL CMS-X-ALPHA TREATMENT OF CH3I PHOTOIONIZATION DYNAMICS - OUTER VALENCE SHELL AND IODINE 4D LEVELS

Continuum multiple scattering (CMS-X alpha) calculations are used to i nvestigate methyl iodide photoionization dynamics. The validity of ato mic versus molecular models of behaviour for the localised iodine 4d a nd 5p (lone pair) electrons, where conflicting experimental interpreta tions have been offered, is examined. A good account of all the availa ble experimental data is obtained. While die calculations provide some limited support for an atomic-like description of the iodine 4d(-1) p henomenological cross-section, the atomic picture has little validity for the 5p(-1) ionization. Variations in both cross-section and beta p arameter which were previously ascribed to the Cooper minimum are iden tified to be more likely a core-valence shell interaction, opening abo ve the 4d(-1) threshold. The other valence shell channels are similarl y affected. Significant l-wave mixing, induced by the molecular anisot ropy, is found to result in the creation of mixed parity final states, contrary to the atomic model. In these circumstances odd and even har monic components can interfere to create orientation (odd harmonic ter ms) in the photoelectron angular distribution (PAD). Molecule frame PA Ds are calculated and compared to experimental data for the ionization of oriented molecules, confirming an intuitive expectation df asymmet ric electron emission from an asymmetric molecule. The CH3 group there fore acts as a non-negligible perturbation, even on quite localised io dine 5p electrons, and the molecular environment is a paramount factor .