Infrared spectra of isolated protonated polycyclic aromatic hydrocarbon molecules

Gas-phase infrared (IR) spectra of larger protonated polycyclic aromatic hydrocarbon molecules, H(+)PAH, have been recorded for the first time. The ions are generated by electrospray ionization and spectroscopically assayed by IR multiple-photon dissociation (IRMPD) spectroscopy in a Fourier transform ion cyclotron resonance mass spectrometer using a free electron laser. IRMPD spectra of protonated anthracene, tetracene, pentacene, and coronene are presented and compared to calculated IR spectra. Comparison of the laboratory IR spectra to an astronomical spectrum of the unidentified IR emission (UIR) bands obtained in a highly ionized region of the interstellar medium provides for the first time compelling spectroscopic support for the recent hypothesis that H(+)PAHs contribute as carriers of the UIR bands.