Formation of s-bearing species by VUV/EUV irradiation of h₂s-containing ice mixtures: Photon energy and carbon source effects

Carbonyl sulfide (OCS) is a key molecule in astrobiology that acts as a catalyst in peptide synthesis by coupling amino acids. Experimental studies suggest that hydrogen sulfide (H₂S), a precursor of OCS, could be present in astrophysical environments. In the present study, we used a microwave-discharge hydrogen-flow lamp, simulating the interstellar UV field, and a monochromatic synchrotron light beam to irradiate CO:H₂S and CO₂:H₂S ice mixtures at 14 K with vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) photons in order to study the effect of the photon energy and carbon source on the formation mechanisms and production yields of S-containing products (CS₂, OCS, SO₂, etc.). Results show that (1) the photo-induced OCS production efficiency in CO:H₂S ice mixtures is higher than that of CO₂:H₂S ice mixtures; (2) a lower concentration of H₂S enhances the production efficiency of OCS in both ice mixtures; and (3) the formation pathways of CS₂ differ significantly upon VUV and EUV irradiations. Furthermore, CS₂ was produced only after VUV photoprocessing of CO:H₂S ices, while the VUV-induced production of SO₂ occurred only in CO₂:H₂S ice mixtures. More generally, the production yields of OCS, H₂S₂, and CS₂ were studied as a function of the irradiation photon energy. Heavy S-bearing compounds were also observed using mass spectrometry during the warm-up of VUV/EUV-irradiated CO:H₂S ice mixtures. The presence of S-polymers in dust grains may account for the missing sulfur in dense clouds and circumstellar environments.