  We present experimental constraints on the insertion of oxygen atoms into
methane to form methanol in astrophysical ice analogs. In gas-phase and
theoretical studies this process has previously been demonstrated to have a
very low or non-existent energy barrier, but the energetics and mechanisms have
not yet been characterized in the solid state. We use a deuterium UV lamp
filtered by a sapphire window to selectively dissociate O2 within a mixture of
O2:CH4 and observe efficient production of CH3OH via O(1D) insertion. CH3OH
growth curves are fit with a kinetic model, and we observe no temperature
dependence of the reaction rate constant at temperatures below the oxygen
desorption temperature of 25K. Through an analysis of side products we
determine the branching ratio of ice-phase oxygen insertion into CH4: ~65% of
insertions lead to CH3OH with the remainder leading instead to H2CO formation.
There is no evidence for CH3 or OH radical formation, indicating that the
fragmentation is not an important channel and that insertions typically lead to
increased chemical complexity. CH3OH formation from O2 and CH4 diluted in a
CO-dominated ice similarly shows no temperature dependence, consistent with
expectations that insertion proceeds with a small or non-existent barrier.
Oxygen insertion chemistry in ices should therefore be efficient under
low-temperature ISM-like conditions, and could provide an important channel to
complex organic molecule formation on grain surfaces in cold interstellar
regions such as cloud cores and protoplanetary disk midplanes.
