Formation of interstellar OCS: Radiation chemistry and IR spectra of precursor ices

Extensive experimental studies have been performed on the solid-state formation of the OCS molecule in proton-irradiated water-free and water-dominated ices containing CO or CO2 as the carbon source and H2S or SO2 as the sulfur source. In each case OCS is readily formed. Production efficiency follows the trends CO > CO2 and H2S > SO2 as C, O- and S-sources, respectively. In water-dominated ices, OCS production appears to be enhanced for CO : H2S reactants. The mechanism of formation of OCS appears to be the reaction of CO with free S atoms produced by fragmentation of the sulfur parent species. While OCS is readily formed by irradiation, it is also the most easily destroyed on continued exposure. In H2O-dominated ices the half-life of H2S, SO2, and OCS is similar to 2 eV molecule(-1), corresponding to similar to 7 million years in a cold dense interstellar cloud environment processed by cosmic-ray protons. The spectral profile of the v(3) band of OCS is highly dependent on temperature and ice composition, and changes with radiation processing. These effects can be used in theoretical modeling of interstellar infrared (IR) spectra; a laboratory spectrum of irradiated H2O : CO : H2S, warmed to 50 K, provides a good fit to the 2040 cm(-1) feature in the W33A spectrum. The identification of OCS in CO2-dominated ices provides a further challenge, due to the overlap of the OCS band with that of CO3 formed from irradiation of the host ice. The two features can be unraveled by a curve-fitting procedure. It is the width of the 2040 cm(-1) band that will help observers determine if features identified in CO2-rich ices are due to OCS or to CO3.