Keywords:Astrochemistry, Hydrogen sulfide, Chemical desorption, Molecular cloud
Hydrogen sulfide (H2S) is one of the most abundant sulfur-bearing species in interstellar molecular clouds, which are the birthplaces of stars and planets. Since H2S cannot be efficiently produced by reactions in the gas phase, it is widely accepted that grain-surface reactions are necessary for the synthesis of H2S in those environments. Nonetheless, H2S has been identified in the gas phase only, indicating that desorption processes for H2S from the grain are required. Since the typical temperature of interstellar grains (10 K) is far below the desorption temperature of H2S (>90 K), some non-thermal desorption mechanism(s) should work well at such low temperatures. In the present study, we performed laboratory experiments on the non-thermal desorption of H2S from the surface of interstellar ice analogues at 10 K. We focus on the so-called chemical desorption, which is proposed to be one of the non-thermal desorption mechanisms induced by the heat of reaction, through the reaction of H2S with atomic hydrogen (H) on amorphous solid water (ASW) as follows: H2S + H -> HS + H2 (1), HS + H -> H2S (2). We observed a clear decrease of H2S after reactions with H by infrared spectroscopy; the desorption efficiency far exceeded that for other non-thermal desorption mechanisms such as photon and cosmic-ray induced desorption in molecular clouds. Since the heat of reactions (1) and (2) exceeds the binding energy of physisorbed H2S and HS on ASW, chemical desorption in principle may occur in both reactions. Nevertheless, we expect that reaction (2) is the dominant process for the chemical desorption of H2S because the heat of reaction (2) is significantly larger than that of reaction (1). Since reaction (2) is the final step for H2S formation interstellar grains, a large fraction of H2S is likely to desorb upon the formation, which does not contradict with the non-detection of H2S in the solid state so far and the detection of abundant H2S in the gas phase. Moreover, the present results have a potential to improve the current chemical modeling studies which typically incorporate the H2S desorption efficiency of 0 to 1%.