Experimental and computational studies of the hydrogenation of carbon disulfide (CS2) on ice analogues

Abstract

Carbon disulfide (CS$_2$) is one of the sulfur-bearing species expected to be present in the interstellar medium (ISM). In this study, we investigated the surface reactions of solid CS$_2$ with hydrogen (H) atoms on amorphous solid water (ASW) using laboratory experiments supported by computational calculations. Our results show that CS$_2$ reacts with H atoms through quantum tunneling in the initial step, followed by successive H addition reactions, with or without activation barriers, on icy surfaces. These processes lead to the formation of several sulfur-bearing species, including hydrogen sulfide (H$_2$S), methyl mercaptan (CH$_3$SH), and small amounts of dithioformic acid (HC(S)SH) and methanedithiol (CH$_2$(SH)$_2$). The observed reactivity of CS$_2$ with H atoms provides a plausible explanation for the non-detection of CS$_2$ in interstellar ices. Furthermore, the efficient hydrogenation of the complex molecules derived from CS$_2$, namely HC(S)SH and CH$_2$(SH)$_2$, suggests that these species could be easily undergone with H atoms to produce other S-bearing species under ISM conditions.

Figures (9)

• Figure 1: Summary of the simplified reaction network obtained in this work. Red arrows indicate irreversible pathways, green arrows indicate barrierless pathways and black dashed lines indicate reactions with a barrier, with the number representing the theoretically derived activation energy. Reactions from right to left occur with release of H2 (hydrogen abstractions). Energies are shown in kJ mol$^{-1}$.
• Figure 2: Variation in the different spectra of the solid CS$_2$ after exposure to H atoms for 3, 60, and 120 min at 10 K. A tiny absorption peak observed at 1304 cm$^{-1}$ is assigned to CH$_4$, which may be one of the products formed from reactions of CS$_2$ with H atoms.
• Figure 3: TPD-QMS spectra of the pre-deposited CS$_2$ with H atoms (solid blue lines), compared with H$_2$ molecules (dashed black lines), on c-ASW at 10 K for up to 2 h: (a) at m/z = 48 (for CH$_3$SH), (b) at m/z = 34 (for H$_2$S), and (c) at m/z = 16 (for CH$_4$).
• Figure 4: FTIR spectrum of the co-deposited CS$_2$ with H atoms (blue line) after 2 hours with a low deposition rate of 0.08 ML.min$^{-1}$ compared to the co-deposited CS$_2$ with H$_2$ (black line) on c-ASW at 10 K. The dashed arrows present the new features formed on the ice surface, they could be derived from the S-bearing species such as H$_2$S, CH$_3$SH, H$_2$CS, and CH$_4$. The solid arrows presents remaining CS$_2$ after 2 hours of the co-deposition.
• Figure 5: Variation of the different spectra of the sample for co-deposited CS$_2$ with H atoms at 10 K, recorded when the surface temperature was gradually warm up from 10 to 170 K.