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-μm 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 utilizes 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 experimenal frequencies. The implications for the Aromatic Infrared Bands, specifically the 3-μm band are discussed. Subject headings: astrochemistry — ISM: molecules — methods: laboratory — techniques: spectroscopic — line: identification 1 https://ntrs.nasa.gov/search.jsp?R=20160001637 2017-11-07T04:09:55+00:00Z

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@inproceedings{Maltseva2015HighresolutionIA, title={High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons: the realm of anharmonicity}, author={Elena Maltseva and Annemieke Petrignani and Alessandra Candian and Cameron J. Mackie and Xinchuan Huang and Timothy J. Lee and A. Tielens and Jos Oomens and Wybren Jan Buma}, year={2015} }