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 mu m that correspond to prominent UIR features. The 6.2 mu m 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 sigma configuration. The 3.5 and 7.7 mu m features correspond to the stretches and scissors motion of the aliphatic CH2 group, which also follow directly from protonation. Other strong bands seen at 6.6 and 6.9 mu m 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.