  We present a model for the formation of large organic molecules in dark
clouds. The molecules are produced in the high density gas-phase that exists
immediately after ice mantles are explosively sublimated. The explosions are
initiated by the catastrophic recombination of trapped atomic hydrogen. We
propose that, in molecular clouds, the processes of freeze-out onto ice
mantles, accumulation of radicals, explosion and then rapid (three-body)
gas-phase chemistry occurs in a cyclic fashion. This can lead to a cumulative
molecular enrichment of the interstellar medium. A model of the time-dependent
chemistries, based on this hypothesis, shows that significant abundances of
large molecular species can be formed, although the complexity of the species
is limited by the short expansion timescale in the gas, immediately following
mantle explosion. We find that this mechanism may be an important source of
smaller organic species, such as methanol and formaldehyde, as well as
precursors to bio-molecule formation. Most significantly, we predict the
gas-phase presence of these larger molecular species in quiescent molecular
clouds and not just dynamically active regions, such as hot cores. As such the
mechanism that we propose complements alternative methods of large molecule
formation, such as those that invoke solid-state chemistry within activated ice
mantles.
