Surface abundance change in vacuum ultraviolet photodissociation of CO ₂ 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( ³P_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 CO ₂, 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.