Vacuum ultraviolet photochemistry of CS₂ isolated in H₂/D₂ matrices at 3. 5 K

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. We performed experimental studies on the vacuum ultraviolet (VUV) photochemistry of CS₂ with H₂ under simulated interstellar conditions. 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. 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. We conclude that the photo-dissociation channel CS₂ C + S₂ initiates the formation of CH₄/CD₄ via several reaction steps between the carbon atom and the H₂/D₂ matrix: C + 3H₂/3D₂ łongrightarrow 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. hν