UV Photolysis of C2H5SH in Solid CO and Ar

Abstract

Ethanethiol (C2H5SH) contains the −SH group, a functional group considered to be crucial for the prebiotic polymerization of amino acids. Ethanethiol is also one of about 250 molecules detected in the interstellar medium or circumstellar shells. We irradiated C2H5SH embedded in carbon monoxide (CO) as well as argon (Ar) ices. Using spectroscopy, we detected methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monosulfide (CS), carbonyl sulfide (OCS), dihydrogen sulfide (SH2), thioformaldehyde (H2CS), ethanethial (CH3CHS), thiirane, ethenethione (H2CCS), ethynethiol (HCCSH), thiirene, ethenedithione (SCCS), methylene radical (CH2), ethylthiyl radical (C2H5S), thioketenyl radical (HCCS), sulfanyl radical (SH), and sulfur atoms. Additionally, we observed OCS luminescence during annealing of the Ar matrix sample, which indicates the existence of a free sulfur atom. All products contain fewer atoms than the parent. CH3CHS was the main organic product in CO; OCS is the only product detected that formed due to a reaction with CO. Quantum chemical computations show that the S–H bond can easily be broken upon electronic excitation. Although more experimental and theoretical data are needed, we suggest that C2H5SH can exist only in interstellar clouds shielded from UV radiation like G+0.693-0.027 but not in Orion KL.