Photoionization in ultraviolet processing of astrophysical ice analogs at cryogenic temperatures

Two recent experimental studies have demonstrated that amino acids or amino acid precursors are generated when astrophysical ice analogs are subjected to ultraviolet (UV) irradiation at cryogenic temperatures. Understanding the complete phenomenology of photoprocessing is critical to elucidating chemical reaction mechanisms that can function within an ice matrix under very cold conditions. Pushing beyond the much better characterized study of photolytic dissociation of chemical bonds through electronic excitation, this work explored the ability of UV radiation present in the interstellar medium to ionize small molecules embedded in ices. Quantum chemical calculations, including bulk solvation effects, were used to study the ionization of hydrogen (H ₂ ), water, and methanol (CH ₃ OH) bound in small clusters of water. Ionization potentials were found to be much smaller in the condensed phase than in the gas phase; even a small cluster can account for large changes in the ionization potentials in ice, as well as the known formation of an OH-H ₃ O ⁺ pair in the case of H ₂ O photoionization. To gauge the impact of photoionization on subsequent grain chemistry, the reaction between OH and CO in the presence of H ₃ O ⁺ was studied and compared with the potential energy surface without hydronium present, which is relevant to chemistry following photolysis. The differences indicate that the reaction is somewhat more likely to proceed to products (H + CO ₂ ) in the case of photoionization.