Spin-forbidden shake-up states of OCS molecule studied by resonant photoelectron spectroscopy

Resonant photoionization of OCS at the sulfur 2p(3/2) --> 4pi* resonance is observed. The spectra show three satellite structures between B ((2)Sigma) and C ((2)Sigma) valence-ionized states. Based on ab initio theoretical calculations, these satellites are assigned to (4)Pi, (2)Pi, and (2)Phi shake-up-like satellites with three open shells, where the shake-up electron occupies the 4pi* orbital. The transition to the spin-forbidden (4)Pi state arises from strong singlet-triplet mixing via the spin-orbit coupling in the intermediate sulfur 2p-4pi* excited state. In order to understand the observed satellite intensities, matrix elements for the Auger-like two-electron process from the intermediate sulfur 2p --> 4pi* excited states to the final valence-ionized states are analytically derived. (C) 2004 Elsevier B.V. All rights reserved.