An Isotope-labeling Study of CO₂Formation Pathways in Molecular Clouds: Ultraviolet Photolysis of CO–H₂O Ice

Abstract

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 H2 18O instead of H2 16O 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 C16O2 and C18O16O, which were generated through the CO–CO and CO–H2O reactions, respectively. The additional detection of C18O2 clearly indicated that the photodissociation and regeneration of CO2 also took place in ice. We analyzed the irradiation-time dependence of the C16O2 and C18O16O column densities in CO-rich ice to determine the effective cross sections of the CO2 formation through the CO–CO ( σ C 16 O 2 ) and CO–H2O ( σ C 18 O 16 O ) reactions simultaneously: σ C 16 O 2 = 3 × 10−21 cm2 and σ C 18 O 16 O = 5 × 10−18 cm2. Despite the rather small value of σ C 16 O 2 compared to σ C 18 O 16 O , the former reaction was found to be nonnegligible in terms of the CO2 yield, even at the H2O-rich condition.