H2O is the most abundant solid component in icy grain mantles and plays a variety of roles for chemical evolution in molecular clouds. Since the observed large abundance of solid H2O cannot be explained by the gas phase synthesis only, H2O is considered to form by grain-surface reactions at low temperatures. Theoretical studies predicted that one of the major routes to the formation of H2O is the reaction of hydroxyl radicals (OH) with H2. However, in the gas phase, this reaction has a large barrier of about 2000 K, suggesting that it does not thermally occur on interstellar grains at as low as 10 K. So we have experimentally examined whether H2O is formed by the reaction via quantum-tunneling on a surface at 10 K. We produced OH by the dissociation of H2O in microwave-induced plasma and cooled it to 100 K. The OH was confirmed to be electronically and vibrationally in the ground state. H2O was effectively formed by the reaction of cold OH with H2 while the formation HDO was much less efficient by the reaction of OH with D2 under the same experimental conditions. The observed mass dependence strongly suggests that these reactions proceed through quantum tunneling. Thus quantum tunneling should play a pivotal role for H2O formation if the reaction of OH with H2 is the dominant process in molecular clouds.

Role of quantum tunneling for the formation of H<sub>2</sub>O by reaction of H<sub>2</sub> with OH on interstellar grains

247th National Meeting (2014)

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Role of quantum tunneling for the formation of H2O by reaction of H2 with OH on interstellar grains

Role of quantum tunneling for the formation of H₂O by reaction of H₂ with OH on interstellar grains