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

¹ Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: w.j.buma@uva.nl
² Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525 ED Nijmegen, The Netherlands
e-mail: a.rijs@science.ru.nl
³ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁴ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands

Received: 5 April 2019
Accepted: 15 July 2019

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 μm 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.