Cracking the puzzle of CO2 formation on interstellar ices

Abstract

Context. Carbon dioxide (CO2) is one of the dominant components of interstellar ices. Recent observations show CO2 exists more abundantly in polar (H2O-dominated) ice than in apolar (H2O-poor) ice. Formation of CO2 ice is primarily attributed to the reaction between CO and OH, which has a barrier. Aims. We investigate the title reaction in H2O ice and CO ice to quantify the efficiency of the reaction in polar ice and apolar ice. Methods. Highly accurate quantum chemical calculations were employed to analyze the stationary points of the potential energy surfaces of the title reaction in the gas phase on H2O and CO clusters. Microcanonical transition state theory was used as a diagnostic tool for the efficiency of the reaction under interstellar medium conditions. We simulated the kinetics of ice chemistry, considering different scenarios involving non-thermal processes and energy dissipation. Results. The CO + OH reaction proceeds through the remarkably stable intermediate HOCO radical. On the H2O cluster, the formation of this intermediate is efficient, but the subsequent reaction leading to CO2 formation is not. Conversely, HOCO formation on the CO cluster is inefficient without external energy input. Thus, CO2 ice cannot be formed by the title reaction alone either on an H2O cluster or a CO cluster. Conclusions. In the polar ice, CO2 ice formation is possible via CO + OH → HOCO followed by HOCO + H → CO2 + H2, as demonstrated by abundant experimental literature. In apolar ice, CO2 formation is less efficient because HOCO formation requires external energy. Our finding is consistent with the JWST observations. Further experimental work using low-temperature OH radicals is encouraged.