Carbonic acid production in H_2O:CO_2 ices. UV photolysis vs. proton bombardment

We have the unique ability to perform ultraviolet photolysis ( ~ 10 eV per photon) and ion irradiation (0.8 MeV p^+) in the same experimental set-up, with ices created under identical conditions. We present experiments that show the formation of carbonic acid (H_2CO_3) from H_2O:CO_2 ice mixtures exposed to either UV photons or high-energy protons. CO and CO_3 were also formed in these experiments. Results show that while H_2CO_3 is readily formed by p^+ bombardment, its formation by UV photolysis is limited by the penetration of UV photons into the ice. H_2CO_3 production pathways are investigated. Intrinsic IR band strengths are determined for eight IR features of H_2CO_3. Implications for ices found in various astrophysical environments are discussed.