The Search for Novel Carbon Oxides Within Irradiated CO₂ Ices: Potential New Parent Species for Cometary Volatiles

Comets are thought to vital information about the chemical inventory of the primordial nebula from which our Solar System formed. As such, understanding their composition provides key insights into how the Solar System formed, and also their potential to exogenously deliver prebiotically important molecules to the early Earth. However, both the cometary ices, as well as the interstellar ices they inherited have undergone extensive chemical altering due to exposure to ionizing radiation (photons, electrons, and energetic ions), which has chemically processed simple volatiles into more complex species. Such radiated ice environments can uniquely form unusual chemical species that are not commonly observed. As cometary surfaces are warmed on approach to the Sun, these parent species may be released and broken down into daughter fragments more commonly observed within the coma of comets. Here we report the findings of extensive computational calculations (using DFT, Møller-Plesset, and Couple Cluster methods) on higher order carbon oxide species (C_xO_y; x=1,2, y=1-8) alongside laboratory irradiation experiments of CO₂-containing ices. We calculated the optimized geometries, harmonic and anharmonic vibrational frequencies, infrared intensities, Raman activities of C_xO_y species using DFT (BLYP, B3LYP, PBE, PBE0, ωB97X-D), ab initio methods [MP2, SCS-MP2, CCSD, CCSD(T), and CCSD(2)_T] and studied the energetics using composite methods (G3MP2, CCCA-CCL, and CCCA-S4). Experimentally, we irradiated various CO₂-containing ices with 2 keV electrons in an Ultra-High Vacuum (UHV) chamber with background pressure ~10^-10 torr. The chemical changes within the ice were monitored using transmission IR spectroscopy, and desorbed species were monitored via mass spectrometry using a high sensitivity quadrupole mass spectrometer. We compare our computational and experimental results to identify unusual carbon oxide species that could help explain perplexing observational data of comet coma compositions, such as the recent detection of molecular oxygen (O₂) at Comet 67P/Churyumov-Gerasimenko.