Anharmonicity in the mid-infrared spectra of polycyclic aromatic hydrocarbons: Molecular beam spectroscopy and calculations

Abstract

Aims. In this work we determine the effects of anharmonicity on the mid-infrared spectra of the linear polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene, tetracene and pentacene recorded using the free electron laser FELIX. Methods. Comparison of experimental spectra obtained under supersonic jet conditions with theoretically predicted spectra was used to show if anharmonicity explicitly needs to be taken into account. Results. Anharmonic spectra obtained using second-order vibrational perturbation theory agree on average within 0.5% of the experimental frequencies. Importantly, they confirm the presence of combination bands with appreciable intensity in the 5-6 $\mu$m region. These combination bands contain a significant fraction of the IR absorption, which scales linearly with the size of the PAH. Detection and assignment of the combination bands are a preliminary indication of the accuracy of far-IR modes in our anharmonic theoretical spectra. Detailed analysis of the periphery-sensitive CH out-of-plane band of naphthalene reveals that there is still room for improvement of the VPT2 approach. In addition, the implications of our findings for the analysis of the aromatic infrared bands are discussed.

arXiv comment10 pages, 3 figures and Appendix. Accepted in Astronomy and Astrophysics

References in corpus (4)

• Mapping PAH sizes in NGC 7023 with SOFIA
• High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons: the realm of anharmonicity
• The 5.25 & 5.7 $\mu$m Astronomical Polycyclic Aromatic Hydrocarbon Emission Features
• High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons in the 3 micron region: role of hydrogenation and alkylation

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• PAH Spectral Diversity in NGC 7027 and the Evolution of Aromatic Carriers
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