  Three structural isomers of the C2H4O2 molecule, namely, methyl formate (MF;
HCOOCH3), acetic acid (AA; CH3COOH), and glycol aldehyde (GA; HOCH2CHO), have
attracted considerable attention as targets for understanding pathways towards
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 (UV) photons and cosmic-ray
analogues. 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 ultra-high-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.
