Electronic spectrum of 9-methylanthracenium radical cation

The predissociation spectrum of the cold, argon-tagged, 9-methylanthracenium radical cation is reported from 8000 cm⁻¹ to 44 500 cm⁻¹. The reported spectrum contains bands corresponding to at least eight electronic transitions ranging from the near infrared to the ultraviolet. These electronic transitions are assigned through comparison with ab initio energies and intensities. The infrared D₁←D₀ transitions exhibit significant vibronic activity, which is assigned through comparison with TD-B3LYP excited state frequencies and intensities, as well as modelled vibronic interactions. Dissociation of 9-methylanthracenium is also observed at high visible-photon energies, resulting in the loss of either CH₂ or CH₃. The relevance of these spectra, and the spectra of other polycyclic aromatic hydrocarbon radical cations, to the largely unassigned diffuse interstellar bands, is discussed.