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.
@article{arxiv.2110.15881,
title = {Methane formation in cold regions from carbon atoms and molecular hydrogen},
author = {Thanja Lamberts and Gleb Fedoseev and Marc van Hemert and Danna Qasim and Ko-Ju Chuang and Julia C. Santos and Harold Linnartz},
journal= {arXiv preprint arXiv:2110.15881},
year = {2022}
}