Inner-shell photoelectron angular distributions from fixed-in-space OCS molecules: comparison between experiment and theory

Photoelectron angular distributions (PADs) for O 1s, C 1s and S 2p(1/2), 2p(3/2) ionization of OCS molecules have been measured in shape resonance regions. These PAD results are compared with the results for O 1s and C 1s ionization of CO molecules, and multi-scattering X alpha (MSX alpha) calculations. The mechanism of the PAD formation both for parallel and perpendicular transitions differs very significantly in these molecules and a step from a two-centre potential (CO) to a three-centre potential (OCS) plays a principal role in electron scattering and the formation of the resulting PAD. For parallel transitions, it is found that for the S 2p and O 1s ionization the photoelectrons are emitted preferentially in a hemisphere directed to the ionized S and O atom, respectively. In OCS O 1s ionization, the S-C fragment plays the role of a strong 'scatterer' for photoelectrons, and in the shape resonance region most intensities of the PADs are concentrated on the region directed to the O atom. The MSXa calculations for perpendicular transitions reproduce the experimental data, but not so well as in the case of parallel transitions. The results of PAD, calculated with different l(max) on different atomic centres, reveal the important role of the d (l = 2) partial wave for the S atom in the partial wave decompositions of photoelectron wavefunctions.