Photodissociation of the acenaphthene, acenaphthylene, and benzofluorene cations

Polycyclic aromatic hydrocarbons (PAHs) are considered to be carriers of the unidentified infrared (UIR) emission bands observed from a variety of interstellar objects. In harsh stellar environments, these PAHs are likely ionized and fragmented by ultraviolet radiation. In this work, photofragmentation pathways and spectroscopic properties were investigated for three PAHs containing pentagonal rings: acenaphthene, acenaphthylene, and benzofluorene.; Photofragmentation routes were characterized for acenaphthene, acenaphthylene, and benzofluorene cations using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Ions were trapped in an ICR cell and irradiated with light from a xenon arc lamp for various lengths of time. The acenaphthene and acenaphthylene cations lost only hydrogen atoms upon UV irradiation, whereas the photofragmentation of benzofluorene involved carbon loss. Infrared and electronic absorption spectra were collected for neutral and cationic benzofluorene trapped in solid argon at 12 K. Infrared absorption bands observed for the benzofluorene cation matched several of the UIR emission bands. Density functional theory calculations yielded optimized geometries, ground-state energies, and vibrational frequencies for each PAH cation. Predicted vibrational frequencies correlated well with those observed in solid argon.; Evidence of an acenaphthylene aggregate ion was observed in FT-ICR studies. Although neutral acenaphthylene dimerization has been well characterized, the structure and properties of the dimer-like ion are unknown. Photofragmentation pathways and predicted structures for the acenaphthylene aggregate ion are reported in this work.