Methanol Formation via the Radical–Radical Reaction of OH and CH ₃ Radicals Undergoing Transient Diffusion on Ice at 10 to 60 K

Abstract Methanol (CH 3 OH) is thought to form on interstellar ice dust via successive hydrogenation reactions. The reaction between CH 3 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 CH 3 and OH radicals on the surface of amorphous solid water, an ice dust analog, at temperatures from 10 to 60 K. In the present experiments, OH radicals were generated by ultraviolet photolysis of water molecules, following which methane (CH 4 ) gas was deposited on the ice substrate. The results show that CH 3 OH was formed on the ice surface through the sequential reactions CH 4 + OH → CH 3 + H 2 O and CH 3 + OH → CH 3 OH, 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 CH 3 due to the heat of the first reaction.