Electronic absorption spectra of protonated anthracenes and phenanthrenes, and their neutrals in neon matrices

Electronic spectra of three isomers of protonated anthracene and five isomers of protonated phenanthrene have been detected in 6 K neon matrices following deposition of mass-selected m/z = 179 cations produced from dihydro-anthracene or -phenanthrene. The cations exhibit moderately intense band systems in the 400-550 nm range. Corresponding neutrals have been observed in the UV. The absorptions are assigned to specific isomers of the protonated species on the basis of time-dependent density functional theory calculations. The astrophysical relevance of protonated anthracenes and phenanthrenes as candidates for carriers of diffuse interstellar bands is discussed.