Photofragmentation of carbonyl sulfide (OCS) following core-shell photoexcitation and photoionization

Mass spectroscopy is used to study the formation of cations and anions after photoexcitation and photoionization of OCS in the vicinity of the S 2p, C 1s and O 1s thresholds. Singly charged cations are produced predominantly at the pi* and Rydberg resonances while dications are major products of dissociative channels above the thresholds. At the S 2p threshold, broad Franck-Condon regions observed for Rydberg excited states manifests an apparent change in molecular bond lengths upon the excitation of S 2p electrons to Rydberg orbitals. Neutral dissociation of the pi*3/2, pi*1/2 and 4S 3/2 excited states is found to compete efficiently with electronic relaxation via Auger processes. The observation of SO+ ions at the pi* resonance indicates a strong Renner-Teller vibronic coupling of bending vibrations in the 4pi* excited states with a bent equilibrium geometry. The anions (S-, C- and O-) are observed both below and above the thresholds. The fact that the anion production is suppressed at shape resonances allows the anions to serve as a suggestive and complementary tool for identifying above-threshold structures in the OCS photoabsorption spectra. Post-collision-interaction effects in OCS and locality of the lowest unoccupied molecular orbital 4pi* at the sulfur atom are also evidenced in the anion spectra.