Surface abundance change in vacuum ultraviolet photodissociation of CO2 and H2O mixture ices.

Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O(P-3(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO2. Two different adsorption states of CO2, i.e., physisorbed CO2 on the surface of amorphous solid water and trapped CO2 in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H2O. Since the absorption cross section of CO2 is smaller than that of H2O at 157 nm, the CO2 surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed.