Anharmonicity and the infrared emission spectrum of highly excited PAHs

TL;DR

This paper develops a computational approach combining vibrational perturbation theory and the Wang-Landau method to accurately simulate the infrared spectra of highly excited polycyclic aromatic hydrocarbons, accounting for anharmonic effects and temperature.

Contribution

It introduces a novel method that fully incorporates anharmonicity and temperature effects into IR spectra calculations for excited PAHs, validated against experimental data.

Findings

Calculated spectra match experimental data in position, width, and intensity.

The method accurately captures anharmonic effects like resonances and overtones.

Validated against pyrene molecule IR spectra.

Abstract

Aims. Infrared (IR) spectroscopy is a powerful tool to study molecules in space. A key issue in such analyses is understanding the effect that temperature and anharmonicity have on different vibrational bands, and thus interpreting the IR spectra for molecules under various conditions. Methods. We combined second order vibrational perturbation theory and the Wang-Landau random walk technique to produce accurate IR spectra of highly excited PAHs. We fully incorporated anharmonic effects, such as resonances, overtones, combination bands, and temperature effects. Results. The results are validated against experimental results for the pyrene molecule (C16H10). In terms of positions, widths, and relative intensities of the vibrational bands, our calculated spectra are in excellent agreement with gas-phase experimental data.