Nonenergetic reactions between atomic hydrogen and molecules on interstellar grain surfaces

Reactions of atomic hydrogen with CO and O₂ on amorphous solid water (ASW), relevant to chemical evolution on cosmic ice dust, were experimentally investigated at around 10 K. Successive addition of hydrogen atoms to CO and O₂ produces H₂CO, CH₃OH, and H₂O₂, H₂O, respectively. At such low temperatures, some of hydrogen additions such as H + CO → HCO proceed via tunneling reactions rather than thermally-activated reactions. Effective reaction rates and isotope effect of the tunneling reactions to produce HCO and H₂O were measured. The surface of ASW was found to enhance the effective rate of hydrogen addition to CO at relatively higher temperatures, namely around 20 K.