Formation of sulfur-bearing complex organic molecules in interstellar ice analogues via UV photochemistry of CH3SH

Sulfur chemistry in the interstellar medium has received renewed attention in recent years, driven by long-standing questions about sulfur depletion between diffuse and dense environments and by recent tentative reports of sulfur-bearing organics such as DMS (CH₃SCH₃) and/or DMDS (CH₃SSCH₃) in exoplanet atmospheres. These topics highlight the need to better understand how simple sulfur-bearing molecules are processed and transformed into more complex species. In this context, methanethiol (CH₃SH), the direct sulfur analogue of methanol (CH₃OH), is an excellent starting point for exploring the formation of more complex sulfur-bearing molecules in interstellar ices.

To investigate this chemistry, we performed UV irradiation experiments on pure CH₃SH ices under ultra-high-vacuum conditions. The processed ices were monitored by reflection-absorption infrared spectroscopy (RAIRS) and analyzed during warm-up using temperature programmed desorption (TPD), allowing the identification of newly formed species and to evaluate how their abundances change with experimental conditions.

We find that UV photochemistry of CH₃SH likely produces radicals such as CH₃, CH₃S, and CH₂SH, which recombine to form a variety of sulfur-bearing complex organic molecules. The detected products include CH₃CH₂SH, CH₃SCH₃ (DMS), CH₃SSCH₃ (DMDS), HSCH₂CH₂SH, and CH₃SCH₂SH. We further show that the product yields depend on the ice temperature, thickness, and irradiation time. These results provide new experimental constraints on the solid-state network of sulfur and offer guidance for future searches for sulfur-bearing complex organic molecules in astrophysical environments.