PHOTOSYNTHESIS OF CARBON DIOXIDE FROM CARBON SURFACES COATED WITH OXYGEN: IMPLICATIONS FOR INTERSTELLAR MOLECULAR CLOUDS AND THE OUTER SOLAR SYSTEM

We investigate via infrared spectroscopy the synthesis of CO2 by ultraviolet irradiation (6.41 eV) of amorphous carbon covered with solid O-2 at 21 K. Oxidation occurs at the O-2-carbon interface promoted by photon excitation or dissociation of O-2 molecules. The CO2 production is linear with photon fluence with a yield of 3.3 +/- 0.3 x 10(-5) CO2 photon(-1); the yield does not decrease at high fluences (at least up to 2 x 10(19) photons cm(-2)) since CO2 is not photodissociated at this photon energy. Replacing oxygen with water ice did not produce CO2 since H2O does not dissociate at this photon energy. The CO2 synthesis process discussed in this Letter does not require H2O or CO and may be important in cold astrophysical environments where O-2 could be locally segregated on carbonaceous grains, such as in molecular clouds and icy objects in the outer solar system.