PHOTOCHEMISTRY OF POLYCYCLIC AROMATIC HYDROCARBONS IN COSMIC WATER ICE: THE ROLE OF PAH IONIZATION AND CONCENTRATION

Abstract

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:H<SUB>2</SUB>O, pyrene:H<SUB>2</SUB>O, and benzo[ghi]perylene:H<SUB>2</SUB>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:H<SUB>2</SUB>O ices were warmed to 270 K, it was not known if they formed during ice irradiation or during warm-up when reactants interact as H<SUB>2</SUB>O 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) <SUB>n</SUB> ] and quinones [PAH(O) <SUB>n</SUB> ] for all PAH:H<SUB>2</SUB>O ices considered here. Little evidence is found for hydrogenation at 14 K, consistent with the findings of Gudipati &amp; Yang. Addition of O and OH to the parent PAH is the dominant photochemical reaction, but PAH erosion to smaller PAHs (producing CO<SUB>2</SUB> and H<SUB>2</SUB>CO) 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) <SUB>n</SUB> and PAH(O) <SUB>n</SUB> to 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 H<SUB>2</SUB>O. PAHs, their photoproducts, and ion-mediated processes should therefore be considered when modeling interstellar ice processes.