Interaction of H₂S with H atoms on grain surfaces under molecular cloud conditions

Abstract

Context. Hydrogen sulfide (H2S) is thought to be efficiently formed on grain surfaces through the successive hydrogenation of sulfur atoms. Its non-detection so far in astronomical observations of icy dust mantles thus indicates that effective destruction pathways must play a significant role in its interstellar abundance. While chemical desorption has been shown to remove H2S very efficiently from the solid phase, in line with H2S gas-phase detections, possible ice chemistry triggered by the related HS radical have been largely disregarded so far, despite it being an essential intermediate in the H2S + H reaction scheme. Aims. We aim to thoroughly investigate the fate of H2S upon H-atom impact under molecular cloud conditions, providing a comprehensive analysis combined with detailed quantification of both the chemical desorption and ice chemistry that ensues. Methods. We performed experiments in an ultrahigh vacuum chamber at temperatures between 10 and 16 K in order to investigate the reactions between H2S molecules and H atoms on interstellar ice analogs. The changes in the solid phase during H-atom bombardment were monitored in situ by means of reflection absorption infrared spectroscopy (RAIRS), and desorbed species were complementarily measured with a quadrupole mass spectrometer (QMS). Results. We confirmed the formation of H2S2 via reactions involving H2S + H and quantified its formation cross section under the employed experimental conditions. Additionally, we directly assessed the chemical desorption of H2S by measuring the gas-phase desorption signals with the QMS, providing unambiguous desorption cross sections. Chemical desorption of H2S2 was not observed. The relative decrease of H2S ices by chemical desorption changed from ~85% to ~74% between temperatures of 10 and 16 K, while the decrease as the result of H2S2 formation was enhanced from ~15% to ~26%, suggesting an increasingly relevant sulfur chemistry induced by HS radicals at warmer environments. The astronomical implications are further discussed.

Formation of S-Bearing Complex Organic Molecules in Interstellar Clouds via Ice Reactions with C2H2, HS, and Atomic H

“…The second strongest desorption feature in Figure 1 peaks at 131 K and contains the mass signals m/z = 32 and 66 (see Figure 5a). Both the peak desorption temperature and the relative intensities of the fragments are in full agreement with the reference values for disulfane (H 2 S 2 ) 50 (Figure 5b). Moreover, its characteristic SH stretching band is observed on the red wing of the H 2 S feature peaking at ∼2491 cm −1 , as shown in Figure 5c.…”

“…Indeed, the formation of this species is unsurprising, as it has been previously shown to form at 10 K in an H 2 S ice exposed to H atoms likely as a result of SH radical recombination. 50 Subsequently to H 2 S 2 , a desorption feature appears at 158 K with contributions from m/z = 46, 47, 58, 59, 60, 61, and 94 (Figure 6, left panels). The mass-to-charge ratios of 92, 118, 120, and 122 were also recorded during the TPD experiment, but no increase in their signal was observed above the instrumental detection limit.…”

“…Parameters Used to Quantify the Products' Relative Abundances with the QMS From the peak signals of m/z = 60 and 62�the largest massto-charge ratios detected for each peak respectively�it is inferred that the product desorbing at ∼117 K is described by the formula C 2 H 4 S, and the one at ∼121 K is described by C 2 H 6 S. Indeed, it is expected that the lighter species with four hydrogen atoms would be more volatile than the fully saturated counterpart, albeit slightly. The contribution of the mass fragment m/z = 32 to these species cannot be measured since it is blended with the desorption profile of H 2 S at ∼85 K (see, e.g., Santos et al 50 ).…”

“…51 This reaction has been shown to enrich the ice mantles with SH radicals, which in turn can kick-start sulfur-bearing ice chemistry at temperatures of relevance to molecular clouds. 50 On a similar context, Laas and Caselli 52 found that a significant portion of the missing sulfur reservoir would consist of simple organosulfur compounds (e.g., OCS, H 2 CS, CS 2 ) locked up in the ices. Solid-state reactions leading to S-bearing molecules could thus play a significant role in unraveling the fate of interstellar sulfur.…”

UV photolysis of thiourea and its N-methylated derivative in cryogenic matrices

“…Nevertheless, they can take part in efficient hydrogenation reactions in an H-rich environment eventually yielding ˙SH and H 2 S (reactions (R13) and (R10)). 50 It is important to note that Krupa et al showed the exclusive presence of the HCN⋯S complex in the Ar matrix after photolyzing HNCS, 51 which is absent in the para -H 2 matrix (possibly due to the diminished cage effect) but likely exist in the Ar matrix as there is a very weak signal at 3217.8 cm −1 , close to the literature value. Another possibility, which cannot be excluded, is the photodecomposition of H 2 N–CN resulting in the formation of isocyano/cyano resonance structures (˙NC ↔ ˙CN, reaction (R14)).…”