Ultraviolet photolysis of CO₂ clathrate hydrate and H₂O–CO₂ mixed ice under ultrahigh vacuum

Photolysis of mixed ices, such as H₂O-CO₂, is a key driver of chemical evolution in planetary, cometary, and interstellar environments. Despite their ubiquitous presence, the photolysis of H₂O-CO₂ ices remains underexplored experimentally, largely due to the significant attenuation of vacuum ultraviolet light caused by the intermixing of H₂O and CO₂, which restricts the formation of new species. Here, we demonstrate two previously unknown photolysis pathways for H₂O-CO₂ ices at 10 K under ultrahigh vacuum (∼10^-10 mbar), revealing differences between bulk and surface photochemistry. In bulk, first, we trapped CO₂ within clathrate hydrate (CH) cages to prevent intermixing with H₂O, then subjected it to photolysis and analyzed it <i>via</i> reflection absorption infrared spectroscopy. Our results demonstrate that photon-induced destruction of the clathrate cages prompts the free CO₂ to migrate into the ice matrix without producing any new photoproducts. In contrast, a mixed solid formed by the simultaneous deposition of residual H₂O on CO₂ ice produces a variety of photoproducts on the surface, which were detected using Cs⁺-based secondary ion mass spectrometry. Photoproducts such as CO, H₂CO₃, and CH₃OH, along with elusive intermediates such as HCO, H₂CO, and HCO₃ were seen on the surface but were not observed in the bulk. These results present a better understanding of the synthesis and chemical evolution of H₂O-CO₂-rich astrophysical environments.