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

Experimental gas phase infrared spectra are presented for four cationic pol yaromatic hydrocarbons (PAHs) incorporating a Five-membered ring, indane, a cenaphthene, fluorene, and fluoranthene. The ions are stored in a quadrupol e 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 allo wed infrared transition in the ion, multiphoton absorption occurs, resultin g in dissociation. Recording the fragment ion yield as a function of FEL wa velength gives a background-free infrared spectrum of the parent ion specie s. Experimental spectra are found to be in good overall agreement with dens ity functional calculations, except for those for the fluoranthene cation.