2013ApJ...773..184M

The formation of sulfur-bearing molecules in interstellar ices was investigated during the irradiation of carbon disulfide (CS₂)-oxygen (O₂) ices with energetic electrons at 12 K. The irradiation-induced chemical processing of these ices was monitored online and in situ via Fourier transform infrared spectroscopy to probe the newly formed products quantitatively. The sulfur-bearing molecules produced during the irradiation were sulfur dioxide (SO₂), sulfur trioxide (SO₃), and carbonyl sulfide (OCS). Formations of carbon dioxide (CO₂), carbon monoxide (CO), and ozone (O₃) were observed as well. To fit the temporal evolution of the newly formed products and to elucidate the underlying reaction pathways, kinetic reaction schemes were developed and numerical sets of rate constants were derived. Our studies suggest that carbon disulfide (CS₂) can be easily transformed to carbonyl sulfide (OCS) via reactions with suprathermal atomic oxygen (O), which can be released from oxygen-containing precursors such as water (H₂O), carbon dioxide (CO₂), and/or methanol (CH₃OH) upon interaction with ionizing radiation. This investigation corroborates that carbonyl sulfide (OCS) and sulfur dioxide (SO₂) are the dominant sulfur-bearing molecules in interstellar ices.