Methylamine Formation on Amorphous Solid Water via the Diffusive CH ₃ + NH ₂ Reaction as Investigated by the Cs ⁺ Ion Pickup Method

Methylamine (CH3NH2) has been detected in the interstellar media and is considered an important precursor of prebiotic molecules. In addition to gas-phase processes and energetic processing of ice, two nonenergetic pathways have been proposed for the production of CH3NH2 on icy grain surfaces: successive hydrogenation reaction of HCN and CH3 + NH2 reaction. In this work, the latter process was experimentally investigated using the Cs+ ion pickup method, which allowed us to detect reactants (CH3 and NH2) and the product (CH3NH2) in situ. The CH3 and NH2 radicals were produced on amorphous solid water photolyzed by ultraviolet photons, where OH radicals abstracted H atoms from CH4 and NH3, respectively. CH3NH2 was produced even at 10 K, most likely as a result of the transient diffusion mechanism, in which the NH2 radical transiently diffuses a significant distance upon formation to encounter a CH3 radical. During warm-up of samples, the CH3NH2 yield increased between 15–30 K, probably due to the thermal diffusion of CH3 radicals facilitating the CH3 + NH2 → CH3NH2 reaction.