Vacuum ultraviolet photochemistry of CS₂ isolated in H₂/D₂ matrices at 3.5 K

Sorbonne Université, CNRS, MONARIS UMR 8233, 75005 Paris, France

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Received: 1 October 2025
Accepted: 20 January 2026

Abstract

Context. The evolution of sulphur-bearing species in the interstellar medium remains to be understood. One hypothesis that addresses the sulphur depletion issue postulates that sulphur-bearing molecules are present in interstellar ices, such as CS₂. The evolution of CS₂ under interstellar conditions has not yet been fully studied.

Aims. We performed experimental studies on the vacuum ultraviolet (VUV) photochemistry of CS₂ with H₂ under simulated interstellar conditions.

Methods. Gas mixtures of CS₂ in either H₂ or D₂ with a relative proportion of 1:1000 were deposited on a gold substrate at 3.5 K. The matrices were irradiated with a VUV lamp, and the formed species were followed at 3.5 K by IR spectroscopy. Temperature programmed desorption (TPD) was also performed to probe desorbing species using quadrupolar mass spectrometry.

Results. The formation of CH₄ (or CD₄ with D₂) and CS was detected by IR spectroscopy at 3.5 K. A slower formation kinetics of CD₄ compared to CH₄ is observed. No S-H bond formation was detected at 3.5 K, whereas the thermal desorption of H₂S/D₂S, CH3SH/CD3SD, and CH4/CD4 occurred during TPD. The desorption of S, S₂, and S₃ was not detected.

Conclusions. We conclude that the photo-dissociation channel CS₂ → C + S₂ C + S₂ initiates the formation of CH4/CD4 via several reaction steps between the carbon atom and the H₂/D₂ matrix: C + 3H₂/3D₂ → CH4/CD4 + 2H/2D. We propose that the H/D generated by this reaction scheme induces the hydrogenation of the remaining sulphur-bearing species (S₂, CS, S, and/or CS₂) during TPD to form H₂S/D₂S and CH3SH/CD3SD. As no S-H bonds are detected at 3.5 K, the reactions H₂ + X, where X = CS, S, and S₂ in ground or excited states, do not seem to be effective. The astronomical relevance of these reaction channels is discussed.