2015ApJ...799...14C

Infrared spectroscopic studies of ultraviolet (UV) irradiated, water-rich, cosmic ice analogs containing small polycyclic aromatic hydrocarbons (PAHs) are described. The irradiation studies of anthracene:H₂O, pyrene:H₂O, and benzo[ghi]perylene:H₂ O ices (14 K) at various concentrations reported by Bouwman et al. are extended. While aromatic alcohols and ketones have been reported in residues after irradiated PAH:H₂O ices were warmed to 270 K, it was not known if they formed during ice irradiation or during warm-up when reactants interact as H₂O sublimes. Recent work has shown that they form in low temperature ice. Using DFT computed IR spectra to identify photoproducts and PAH cations, we tentatively identify the production of specific alcohols [PAH(OH)_n] and quinones [PAH(O)_n] for all PAH:H₂O ices considered here. Little evidence is found for hydrogenation at 14 K, consistent with the findings of Gudipati & Yang. Addition of O and OH to the parent PAH is the dominant photochemical reaction, but PAH erosion to smaller PAHs (producing CO₂ and H₂CO) is also important. DFT spectra are used to assess the contribution of PAH-related species to interstellar absorption features from 5 to 9 µm. The case is made that PAH cations are important contributors to the C2 component and PAH(OH)_nand PAH(O)_nto the C5 component described by Boogert et al. Thus, interstellar ices should contain neutral and ionized PAHs, alcohols, ketones and quinones at the ∼2%-4% level relative to H₂O. PAHs, their photoproducts, and ion-mediated processes should therefore be considered when modeling interstellar ice processes.