2021ApJ...921L..13I

Three structural isomers of the C₂H₄O₂ molecule, namely, methyl formate (MF; HCOOCH₃), acetic acid (CH₃COOH), and glycolaldehyde (HOCH₂CHO), have attracted considerable attention as targets for understanding pathways toward molecular complexity in the interstellar medium (ISM). Among these isomers, MF is decisively abundant in various astronomical objects. For various formation pathways of MF, surface reactions on cosmic dust would play an important role. However, when compared to observations, the formation of MF has been found to be relatively inefficient in laboratory experiments in which methanol (CH₃OH)-dominant ices were processed by ultraviolet photons and cosmic-ray analogs. Here, we show experimental results on the effective formation of MF by the photolysis of CH₃OH on water ice at 10 K. We found that the key parameter leading to the efficient formation of MF is the supply of OH radicals by the photolysis of H₂O, which significantly differs from CH₃OH-rich experimental conditions. Moreover, using an ultrahigh-sensitivity surface analysis method, we succeeded in constraining the decisive formation pathway of MF via the photolysis of methoxymethanol (MM; CH₃OCH₂OH), which would improve our current understanding of chemical evolution in the ISM.