Formation of dimethyl sulfide (CH3SCH3) and ethanethiol (CH3CH2SH) in interstellar analog ices of methane (CH4) and hydrogen sulfide (H2S)

Abstract

Hitherto unidentified abiotic formation pathways leading to the organosulfur molecules ethanethiol (C₂H₅SH), methanethiol (CH₃SH) and, dimethyl sulfide (CH₃SCH₃) were investigated through a series of laboratory simulation experiments. Interstellar analog ices of methane (CH₄) and hydrogen sulfide (H₂S) were exposed to proxies of galactic cosmic rays (GCRs) in the form of energetic electrons released in the GCR track in interstellar ices simulating typical cold molecular cloud lifetimes of a few 10⁶ to 10⁷ years. During the temperature-programmed desorption phase, the molecules subliming fractionally from the ice mixtures were photoionized with vacuum ultraviolet (VUV) photons at energies both above and below the adiabatic ionization energies of the product molecules of interest. Exploiting photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) and isotopically labelled ice experiments, the reaction products were selectively photoionized to discriminate between isomers. Ethane (C₂H₆) and methanethiol (CH₃SH), as first-generation irradiation products, along with second-generation dimethyl sulfide (CH₃SCH₃), were identified via infrared spectroscopy and PI-ReToF-MS. The formation of ethanethiol (C₂H₅SH) was further confirmed by matching the photoionization efficiency (PIE) curve to the experimental PI-ReToF-MS data. Our findings instigate a deeper understanding of interstellar sulfur chemistry linking interstellar and cometary ices to the gas-phase detection of sulfur bearing organics in star-forming regions.