Photochemistry of coronene in cosmic water ice analogs at different concentrations

This work presents the photochemistry of ultraviolet (UV) irradiated coronene in water ices at 15 K, studied using mid-infrared Fourier transform (FTIR) spectroscopy for C₂₄H₁₂:H₂O at concentrations of (1:50), (1:150), (1:200), (1:300) and (1:400). Previous UV irradiation studies of anthracene:H₂O, pyrene:H₂O and benzo[ghi]perylene:H₂O ices at 15 K have shown that aromatic alcohols and ketones, as well as CO₂ and H₂CO are formed at very low temperatures. Like-wise, here, in addition to the coronene cation, hydroxy-, keto-, and protonated coronene (coronene-H⁺) are formed. The rate constants for the decay of neutral coronene and for the formation of photoproducts have been derived. It is shown that PAHs and their UV-induced PAH:H₂O photoproducts have mid-infrared spectroscopic signatures in the 5-8 microm region that can contribute to the interstellar ice components described by Boogert et al. (2008) as C1-C5. Our results suggest that oxygenated and hydrogenated PAHs could be in UV-irradiated regions of the ISM where water-rich ices are important.