  Oxygen atom addition and insertion reactions may provide a pathway to
chemical complexity in ices that are too cold for radicals to diffuse and
react. We have studied the ice-phase reactions of photo-produced oxygen atoms
with C2 hydrocarbons under ISM-like conditions. The main products of oxygen
atom reactions with ethane are ethanol and acetaldehyde; with ethylene are
ethylene oxide and acetaldehyde; and with acetylene is ketene. The derived
branching ratio from ethane to ethanol is ~0.74 and from ethylene to ethylene
oxide is ~0.47. For all three hydrocarbons there is evidence of an effectively
barrierless reaction with O(^1D) to form oxygen-bearing organic products; in
the case of ethylene, there may be an additional barriered contribution of the
ground-state O(^3P) atom. Thus, oxygen atom reactions with saturated and
unsaturated hydrocarbons are a promising pathway to chemical complexity even at
very low temperatures where the diffusion of radical species is thermally
inaccessible.
