The Infrared Spectrum of Protonated Naphthalene and Its Relevance for the Unidentified Infrared Bands

The gas phase infrared spectrum of protonated naphthalene is measured in a cold supersonic beam using mass-selected photodissociation spectroscopy and the messenger atom method. Sharp vibrational band structure is measured throughout the 800–3200 cm −1 region for comparison to the unidentified infrared bands (UIRs). Protonated naphthalene exhibits strong bands at 3.5, 6.2, 7.7, and 8.6 μm that correspond to prominent UIR features. The 6.2 μm feature provides one of the first examples in laboratory spectra for an intense vibrational band at this wavelength. It arises from an "allyl-type" carbon ring distortion, which is a direct consequence of the perturbation on aromatic rings resulting from protonation in the σ configuration. The 3.5 and 7.7 μm features correspond to the stretches and scissors motion of the aliphatic CH 2 group, which also follow directly from protonation. Other strong bands seen at 6.6 and 6.9 μm do not coincide with the strongest UIR features. Protonated naphthalene itself is not a perfect match for the UIR spectra, but it strongly suggests that larger H + PAH species are prominent among the UIR carriers.