An Isotope-labeling Study of CO2 Formation Pathways in Molecular Clouds: Ultraviolet Photolysis of CO-H2O Ice

With the use of reflection-absorption infrared spectroscopy, we investigated the binary ice of CO and H2O at 10 K under ultraviolet irradiation as an analog of the interstellar ice mantle. We employed (H2O)-O-18 instead of (H2O)-O-16 to distinguish the formation pathways of CO2, and prepared mixed-ice samples under CO- and H2O-rich conditions as well as bilayered ice, where CO was condensed on H2O ice. After the ultraviolet irradiation onto any sample, we detected infrared absorption bands of (CO2)-O-16 and (COO)-O-18-O-16, which were generated through the CO-CO and CO-H2O reactions, respectively. The additional detection of (CO2)-O-18 clearly indicated that the photodissociation and regeneration of CO2 also took place in ice. We analyzed the irradiation-time dependence of the (CO2)-O-16 and (COO)-O-18-O-16 column densities in CO-rich ice to determine the effective cross sections of the CO2 formation through the CO-CO (sC(16)O(2)) and CO-H2O (sC(18)O(16)O) reactions simultaneously: sC(16)O(2) = 3x10(-21) cm(2) and sC(18)O(16)O = 5x10(-1)8 cm2. Despite the rather small value of sC(16)O(2) compared to sC(18)O(16)O, the former reaction was found to be nonnegligible in terms of the CO2 yield, even at the H2O-rich condition.