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 CH${}_4$ 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 H${}_2$O ice. These experiments lead to the formation of methane isotopologues detected in situ through reflection absorption infrared spectroscopy. Most notably, CH${}_4$ is formed in an experiment combining C atoms with H${}_2$ on amorphous solid water, albeit slower than in experiments with H atoms present. Furthermore, CH${}_2$D${}_2$ is detected in an experiment of C atoms with H${}_2$ and D${}_2$ on H${}_2$O ice. CD${}_4$, 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 + H${}_2$ -> CH${}_2$ can take place, although not barrierless in the presence of water, b) the reaction CH + H${}_2$ -> CH${}_3$ is barrierless, but has not yet been included in astrochemical models, c) the reactions CH${}_2$ + H${}_2$ -> CH${}_3$ + H and CH${}_3$ + H${}_2$ -> CH${}_4$ + 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.

Submitted to ApJ

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• An Ice Age JWST inventory of dense molecular cloud ices
• Laboratory and Computational Studies of Interstellar Ices
• CoCCoA: Complex Chemistry in hot Cores with ALMA. Selected oxygen-bearing species
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• Modelling methanol and hydride formation in the JWST Ice Age era
• CoCCoA: Complex Chemistry in hot Cores with ALMA. The chemical evolution of acetone from ice to gas
• Hydrogenation of acetaldehyde on interstellar ice analogs reveals limited destruction
• Investigating the impact of reactions of C and CH with molecular hydrogen on a glycine gas-grain network
• Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon