Methane formation in cold regions from carbon atoms and molecular hydrogen
Abstract
Methane is typically thought to be formed in the solid state on the surface of cold interstellar icy grain mantles via the successive atomic hydrogenation of a carbon atom. In the current work we investigate the potential role of molecular hydrogen in the CH4 reaction network. We make use of an ultra-high vacuum cryogenic setup combining an atomic carbon atom beam and both atomic and/or molecular beams of hydrogen and deuterium on a H2O ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH4 is formed in an experiment combining C atoms with H2 on amorphous solid water, albeit slower than in experiments with H atoms present. Furthermore, CH2D2 is detected in an experiment of C atoms with H2 and D2 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 chemical insights. This leads to the following conclusions: a) the reaction C + H2 -> CH2 can take place, although not barrierless in the presence of water, b) the reaction CH + H2 -> CH3 is barrierless, but has not yet been included in astrochemical models, c) the reactions CH2 + H2 -> CH3 + H and CH3 + H2 -> 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.
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