Methane Formation in Cold Regions from Carbon Atoms and Molecular Hydrogen

Methane is typically thought to be formed in the solid state on top of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we investigate the role of molecular hydrogen in the CH4 reaction network. We make use of an ultrahigh vacuum cryogenic setup combining an atomic carbon atom beam with atomic and/or molecular beams of hydrogen and deuterium on a water ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH4 is experimentally formed by combining C atoms with only H-2 on amorphous solid water, albeit more slowly than in experiments where H atoms are also present. Furthermore, CH2D2 is detected in an experiment involving C atoms with H-2 and D-2 on H2O ice. CD4, however, is only formed when D atoms are present in the experiment. These findings have been rationalized by means of computational and theoretical chemical insights. This leads to the following conclusions: (a) the reaction C + H-2 -> CH2 takes place, although it is not barrierless for all binding sites on water, (b) the reaction CH + H-2 -> CH3 is barrierless, but has not yet been included in astrochemical models, (c) the reactions CH2 + H-2 -> CH3 + H and CH3 + H-2 -> CH4 + H can take place only via a tunneling mechanism, and (d) molecular hydrogen possibly plays a more important role in the solid-state formation of methane than assumed so far.