Photoionization of Benzene and Small Polycyclic Aromatic Hydrocarbons in Ultraviolet‐processed Astrophysical Ices: A Computational Study

Abstract

We employed density functional theory (DFT) calculations to model the photoionization behavior ofbenzene and small polycyclic aromatic hydrocarbons when they are embedded in a matrix of water icein order to investigate issues raised by recent experimental work by Gudipati and Allamandola. Theionization energies of benzene, naphthalene, anthracene, and pyrene were found to be lowered by1.5-2.1 eV in water ice. Low-lying vertical electronic excitation energies were computed withtime-dependent DFT for both neutral and ionized species and are found in both cases to be remarkablyunaffected by the ice matrix. Chemical behavior in ultraviolet-photoprocessed ices is alsodiscussed, with a focus on electron recombination and pathways leading to phenol and analogousproducts.