  Carbonyl sulfide (OCS) is an abundant sulfur (S)-bearing species in the
interstellar medium. It is present not only in the gas phase, but also on
interstellar grains as a solid; therefore, OCS very likely undergoes
physicochemical processes on icy surfaces at very low temperatures. The present
study experimentally and computationally investigates the reaction of solid OCS
with hydrogen (H) atoms on amorphous solid water at low temperatures. The
results show that the addition of H to OCS proceeds via quantum tunneling, and
further addition of H leads to the formation of carbon monoxide (CO), hydrogen
sulfide (H2S), formaldehyde (H2CO), methanol (CH3OH) and thioformic acid
(HC(O)SH). These experimental results are explained by our quantum chemical
calculations, which demonstrate that the initial addition of H to the S atom of
OCS is the most predominant, leading to the formation of OCS-H radicals. Once
the formed OCS-H radical is stabilized on ice, further addition of H to the S
atom yields CO and H2S, while that to the C atom yields HC(O)SH. We have also
confirmed, in a separate experiment, the HC(O)SH formation by the HCO reactions
with the SH radicals. The present results would have an important implication
for the recent detection of HC(O)SH toward G+0.693-0.027.
