High-resolution IR absorption spectroscopy of polycyclic aromatic
  hydrocarbons: the realm of anharmonicity
  We report on an experimental and theoretical investigation of the importance
of anharmonicity in the 3 micron CH stretching region of Polycyclic Aromatic
Hydrocarbon (PAH) molecules. We present mass-resolved, high-resolution spectra
of the gas-phase cold (~4K) linear PAH molecules naphthalene, anthracene, and
tetracene. The measured IR spectra show a surprisingly high number of strong
vibrational bands. For naphthalene, the observed bands are well separated and
limited by the rotational contour, revealing the band symmetries. Comparisons
are made to the harmonic and anharmonic approaches of the widely used Gaussian
software. We also present calculated spectra of these acenes using the
computational program SPECTRO, providing anharmonic predictions enhanced with a
Fermi-resonance treatment that utilises intensity redistribution. We
demonstrate that the anharmonicity of the investigated acenes is strong,
dominated by Fermi resonances between the fundamental and double combination
modes, with triple combination bands as possible candidates to resolve
remaining discrepancies. The anharmonic spectra as calculated with SPECTRO lead
to predictions of the main modes that fall within 0.5% of the experimental
frequencies. The implications for the Aromatic Infrared Bands, specifically the
3 micron band are discussed.
