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

## TL;DR

This study combines experimental and theoretical methods to analyze anharmonic effects in the mid-infrared spectra of PAHs, revealing the importance of combination bands and improving spectral predictions.

## Contribution

It demonstrates that second-order vibrational perturbation theory accurately predicts anharmonic spectra and highlights the significance of combination bands in PAH IR spectra.

## Key findings

- Anharmonic spectra agree within 0.5% of experimental frequencies
- Combination bands contribute significantly to IR absorption in 5-6 μm region
- Size of PAH correlates with intensity of combination bands

## 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.