Vacuum ultraviolet photochemistry of CS ₂ isolated in H ₂ /D ₂ matrices at 3.5 K

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 CS 2 . The evolution of CS 2 under interstellar conditions has not yet been fully studied. Aims . We performed experimental studies on the vacuum ultraviolet (VUV) photochemistry of CS 2 with H 2 under simulated interstellar conditions. Methods . Gas mixtures of CS 2 in either H 2 or D 2 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 CH 4 (or CD 4 with D 2 ) and CS was detected by IR spectroscopy at 3.5 K. A slower formation kinetics of CD 4 compared to CH 4 is observed. No S-H bond formation was detected at 3.5 K, whereas the thermal desorption of H 2 S/D 2 S, CH3SH/CD3SD, and CH4/CD4 occurred during TPD. The desorption of S, S 2 , and S 3 was not detected. Conclusions . We conclude that the photo-dissociation channel CS 2 → C + S 2 C + S 2 initiates the formation of CH4/CD4 via several reaction steps between the carbon atom and the H 2 /D 2 matrix: C + 3H 2 /3D 2 → CH4/CD4 + 2H/2D. We propose that the H/D generated by this reaction scheme induces the hydrogenation of the remaining sulphur-bearing species (S 2 , CS, S, and/or CS 2 ) during TPD to form H 2 S/D 2 S and CH3SH/CD3SD. As no S-H bonds are detected at 3.5 K, the reactions H 2 + X, where X = CS, S, and S 2 in ground or excited states, do not seem to be effective. The astronomical relevance of these reaction channels is discussed.