Vacuum ultraviolet photochemistry of CS_2 isolated in H_2/D_2 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 CS_2. The evolution of CS_2 under interstellar conditions has not yet been fully studied. We performed experimental studies on the vacuum ultraviolet (VUV) photochemistry of CS_2 with H_2 under simulated interstellar conditions. 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. 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_2S/D_2S, CH_3SH/CD_3SD, and CH_4/CD_4 occurred during TPD. The desorption of S, S_2, and S_3 was not detected. We conclude that the photo-dissociation channel CS_2 C $+$ S_2 initiates the formation of CH_4/CD_4 via several reaction steps between the carbon atom and the H_2/D_2 matrix: C + 3H_2/3D_2 łongrightarrow CH_4/CD_4 + 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_2S/D_2S and CH_3SH/CD_3SD. 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. hν