Methanol Formation via the Radical-radical Reaction of OH and CH3 Radicals Undergoing Transient Diffusion on Ice at 10 to 60 K

Methanol (CH3OH) is thought to form on interstellar ice dust via successive hydrogenation reactions. The reaction between CH3 and OH radicals could also conceivably generate methanol at temperatures above approximately 20 K, at which temperature hydrogen atoms will not adhere to the ice surface. However, this process has not been verified by well-controlled experiments. Using a newly-developed Cs+ ion pickup technique, the authors investigated the reaction between CH3 and OH radicals on the surface of amorphous solid water, an ice dust analogue, at temperatures from 10 to 60 K. In the present experiments, OH radicals were generated by UV photolysis of water molecules, following which methane (CH4) gas was deposited on the ice substrate. The results show that CH3OH was formed on the ice surface through the sequential reactions CH4 + OH -> CH3 + H2O and CH3 + OH -> CH3OH even at 10 K. Considering the very low surface coverage of reactants in the experimental condition, the second reaction was found to occur as a result of transient diffusion of CH3 due to the heat of the first reaction.