Chemical Pathways of SO₂ with Hydrogen Atoms on Interstellar Ice Analogues

Abstract Sulfur dioxide (SO 2 ) is a sulfur-containing molecule expected to exist as a solid in the interstellar medium. In this study, we have performed laboratory experiments and computational studies on the surface reactions of solid SO 2 with hydrogen atoms on amorphous solid water (ASW) at low temperatures. After 40 minutes of exposure of SO 2 deposited on ASW to H atoms, approximately 80% of the solid SO 2 was lost from the substrate at 10–40 K, and approximately 50% even at 60 K, without any definite detection of reaction products. Quantum chemical calculations suggest that H atoms preferentially add to the S atom of solid SO 2 , forming the HSO 2 radical. Further reactions of the HSO 2 radical with H atoms result in the formation of several S-bearing species, including HS(O)OH, the S(O)OH radical, HO–S–OH, HS–OH, and H 2 S. In codeposition experiments involving H and SO 2 , we have confirmed the formation of H 2 S, HS(O)OH, and/or HO–S–OH. However, the yields of these S-bearing species are insufficient to account for the complete loss of the initial SO 2 reactant. These findings suggest that some products are desorbed into the gas phase upon formation. This study indicates that a portion of the SO 2 in ice mantles may remain unreacted, avoiding hydrogenation, while the remainder is converted into other species, some of which may be subject to chemical desorption.