Vacuum ultraviolet photochemistry of CS

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 $\xrightarrow{h\nu}$ 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.