SINGLE-PHOTOIONIZATION, DOUBLE-PHOTOIONIZATION, AND TRIPLE-PHOTOIONIZATION CROSS-SECTIONS OF CARBONYL SULFIDE (OCS) AND IONIC FRAGMENTATION OF OCS+, OCS2+, AND OCS3+

Single-, double-, and triple-photoionization processes of carbonyl sulfide (OCS) have been studied in the photon-energy region of 20-100 eV by use of time-of-flight mass spectrometry and a photoion-photoion coincidence method together with synchrotron radiation. The single (sigma+), double (sigma2+), and triple (sigma3+) photoionization cross sections of OCS are determined; the double-photoionization cross section is found to be considerably larger at hnu = 60 eV, giving a ratio sigma2+/sigma+ of 0.34, than found from previous observations of other molecules (SO2 and CH4). The ratio is in close agreement with that for the isoelectronic zinc atom. Ionic fragmentation ratios of the parent OCS+ and OCS2+ ions are determined separately at the excitation energies where the single and double photoionization takes place concomitantly. The results show that both the (meta)stable OCS+ and OCS2+ ions amount to about 10-15% at higher photon energies in the respective single- and double-photoionization processes. Ionic branching ratios and the partial cross sections for the individual ions produced from the parent OCS+ and OCS2+ ions are also presented. High-lying electronic states of the OCS+ and OCS2+ precursors are also observed.