  Methoxymethanol (CH3OCH2OH, MM) has been identified through gas-phase
signatures in both high- and low-mass star-forming regions. This molecule is
expected to form upon hydrogen addition and abstraction reactions in CO-rich
ice through radical recombination of CO hydrogenation products. The goal of
this work is to investigate experimentally and theoretically the most likely
solid-state MM reaction channel -- the recombination of CH2OH and CH3O radicals
-- for dark interstellar cloud conditions and to compare the formation
efficiency with that of other species that were shown to form along the
CO-hydrogenation line. Hydrogen atoms and CO or H2CO molecules are co-deposited
on top of the predeposited H2O ice to mimic the conditions associated with the
beginning of 'rapid' CO freeze-out. Quadrupole mass spectrometry is used to
analyze the gas-phase COM composition following a temperature programmed
desorption. Monte Carlo simulations are used for an astrochemical model
comparing the MM formation efficiency with that of other COMs. Unambiguous
detection of newly formed MM has been possible both in CO+H and H2CO+H
experiments. The resulting abundance of MM with respect to CH3OH is about 0.05,
which is about 6 times less than the value observed toward NGC 6334I and about
3 times less than the value reported for IRAS 16293B. The results of
astrochemical simulations predict a similar value for the MM abundance with
respect to CH3OH factors ranging between 0.06 to 0.03. We find that MM is
formed by co-deposition of CO and H2CO with H atoms through the recombination
of CH2OH and CH3O radicals. In both the experimental and modeling studies, the
efficiency of this channel alone is not sufficient to explain the observed
abundance of MM. These results indicate an incomplete knowledge of the reaction
network or the presence of alternative solid-state or gas-phase formation
mechanisms.
