Electron Irradiation of H2O Ice on Amorphous Carbon Films: Temperature Dependence and Interfacial CO2 Formation Pathways

The chemical evolution of interstellar icy grain mantles driven by cosmic rays and secondary electrons plays a pivotal role in the molecular complexity of interstellar environments. While bulk ice chemistry has been extensively studied, the coupled physical and chemical processes occurring at the interface between icy mantles and refractory carbonaceous dust grains remain less understood. In Chuang et al. (2023)^[1] and Lee et al. (2026)^[2], we have processed high-energy photons, including X-ray and EUV photon, on H₂O-covered amorphous carbon. Both results implied that the formation of CO₂ bypassed through the intermediate CO phase which is commonly found in ice chemistry. Therefore, in this study, we investigate the effects of energetic electron irradiation on thin H₂O ice layers deposited on isotopically labeled amorphous carbon (a-¹³C) and hydrogenated amorphous carbon (a-¹³C:H) dust analogues.

To systematically examine whether the CO molecules act as the intermediate products of CO₂ formation, irradiation experiments were performed at different temperature ranging from 13 K to 90 K under ultra-high vacuum (UHV) condition. Given that the thermal desorption temperature of CO is approximately 30 K, irradiation above this threshold leads to a lack of CO survival on the dust surface, decreasing the efficiency of CO oxidation to CO₂. Via this method, we provide evidence to exclude the necessity of CO molecules in the formation pathway of CO_2. These results demonstrate that CO₂ formation occurred as long as H₂O ice covers the amorphous carbon substrate, crucially providing another chemical pathway to produce CO₂ ice in higher-temperature (>30 K) environment within the interstellar clouds.