High-resolution IR absorption spectroscopy of polycyclic aromatic hydrocarbons: the realm of anharmonicity

TL;DR

This study combines experimental high-resolution IR spectroscopy and advanced anharmonic theoretical calculations to analyze PAH molecules, revealing strong anharmonic effects and Fermi resonances that influence the interpretation of aromatic infrared bands.

Contribution

It provides the first detailed comparison of experimental spectra with anharmonic predictions for PAHs, demonstrating the importance of anharmonicity and Fermi resonances in IR spectral analysis.

Findings

Anharmonic spectra match experimental data within 0.5%

Strong Fermi resonances dominate anharmonicity in PAHs

High-resolution spectra reveal detailed vibrational band symmetries

Abstract

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.