Photochemistry of the PAH pyrene in water ice: the case for ion-mediated solid-state astrochemistry

Context. Icy dust grains play an important role in the formation of complex molecules in the interstellar medium ( ISH). Laboratory studies have mainly focused on the physical interactions and chemical pathways in ices containing rather simple molecules, such as H2O, CO, CO2, CH4, and CH3OH. Observational studies show that polycyclic aromatic hydrocarbons (PAHs) are also abundantly present in the ISM in the gas phase. It is likely that these non-volatile species also freeze-out onto dust grains and participate in the astrochemical solid-state network, but additional experimental PAH ice studies are largely lacking. Aims. The study presented here focuses on a rather small PAH, pyrene (C16H10), and aims to understand and quantify photochemical reactions of PAHs in interstellar ices upon vacuum ultraviolet (VUV) irradiation as a function of astronomically relevant parameters. Methods. Near UV/VIS spectroscopy is used to track the in situ VUV driven photochemistry of pyrene containing ices at temperatures ranging from 10 to 125 K. Results. The main photoproducts of VUV photolyzed pyrene ices are spectroscopically identified and their band positions are listed for two host ices, H2O and CO. Pyrene ionization is found to be most efficient in H2O ices at low temperatures. The reaction products, triplet pyrene and the 1-hydro-1-pyrenyl radical are most efficiently formed in higher temperature water ices and in low temperature CO ice. Formation routes and band strength information of the identified species are discussed. Additionally, the oscillator strengths of Py, Py center dot+, and PyH center dot are derived and a quantitative kinetic analysis is performed by fitting a chemical reaction network to the experimental data. Conclusions. Pyrene is efficiently ionized in water ice at temperatures below 50 K. Hydrogenation reactions dominate the chemistry in low temperature CO ice with trace amounts of water. The results are placed in an astrophysical context by determining the importance of PAH ionization in a molecular cloud. We conclude that the rate of pyrene ionization in water ice mantles is comparable to the rate of photodesorption of H2O ice. The photoprocessing of a sample PAH in ice described in this manuscript indicates that PAH photoprocessing in the solid state should also be taken into account in astrochemical models.