Gas phase infrared spectroscopy of cationic indane, acenaphthene, fluorene, and fluoranthene

Experimental gas phase infrared spectra are presented for four cationic polyaromatic hydrocarbons (PAHs) incorporating a Five-membered ring, indane, acenaphthene, fluorene, and fluoranthene. The ions are stored in a quadrupole ion trap and irradiated with the intense narrow-band infrared light of a free electron laser (FEL). When the wavelength is in resonance with an allowed infrared transition in the ion, multiphoton absorption occurs, resulting in dissociation. Recording the fragment ion yield as a function of FEL wavelength gives a background-free infrared spectrum of the parent ion species. Experimental spectra are found to be in good overall agreement with density functional calculations, except for those for the fluoranthene cation.