2018ApJ...852...70B

The underlying formation mechanisms of complex organic molecules (COMs)-in particular, structural isomers-in the interstellar medium (ISM) are largely elusive. Here, we report new experimental findings on the role of methanol (CH₃OH) and methane (CH₄) ices in the synthesis of two C₂H₆O isomers upon interaction with ionizing radiation: ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃). The present study reproduces the interstellar abundance ratios of both species with ethanol to dimethyl ether branching ratios of (2.33 ± 0.14):1 suggesting that methanol and methane represents the key precursor to both isomers within interstellar ices. Exploiting isotopic labeling combined with reflectron time-of-flight mass spectrometry (Re-TOF-MS) after isomer selective vacuum ultra-violet (VUV) photoionization of the neutral molecules, we also determine the formation mechanisms of both isomers via radical-radical recombination versus carbene (CH₂) insertion with the former pathway being predominant. Formation routes to higher molecular weight reaction products such as ethylene glycol (HOCH₂CH₂OH), dimethyl peroxide (CH₃OOCH₃), and methoxymethanol (CH₃OCH₂OH) are discussed briefly as well.