Chemical pathways of SO2 with hydrogen atoms on interstellar ice analogues

TL;DR

This study investigates the surface reactions of solid SO2 with hydrogen atoms on interstellar ice analogues, revealing partial hydrogenation, formation of sulfur-bearing species, and possible chemical desorption processes relevant to interstellar chemistry.

Contribution

It combines laboratory experiments and computational analyses to elucidate the reaction pathways of SO2 with H atoms on icy surfaces, highlighting new reaction products and desorption mechanisms.

Findings

Approximately 80% of SO2 was lost after H exposure at 10-40 K.

Formation of sulfur-bearing species such as HSO2, H2S, and HS(O)OH was observed.

Some reaction products likely desorb into the gas phase, explaining SO2 loss.

Abstract

Sulfur dioxide (SO2) is a sulfur-containing molecule expected to exist as a solid in the interstellar medium (ISM). In this study, we performed laboratory experiments and computational analyses on the surface reactions of solid SO2 with hydrogen atoms on amorphous solid water (ASW) at low temperatures. After 40 min of exposure of SO2 deposited on ASW to H atoms, approximately 80% of the solid SO2 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 SO2, forming the HSO2 radical. Further reactions of the HSO2 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 H2S. In codeposition experiments involving H and SO2, we confirmed the formation of H2S, HS(O)OH, and/or HO-S-OH. However, the yields of these S-bearing species were insufficient to account for the complete loss of the initial SO2 reactant. These findings suggest that some products desorbed into the gas phase upon formation. This study indicates that a portion of SO2 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.