Anharmonicity and the infrared emission spectrum of highly excited polycyclic aromatic hydrocarbons

¹ Leiden University, Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: chen@strw.leidenuniv.nl
² School of Engineering Sciences in Chemistry, Biotechnology and Health, Department of Theoretical Chemistry & Biology, Royal Institute of Technology, 10691 Stockholm, Sweden
³ NASA Ames Research Center, Moffett Field, CA 94035-1000, USA

Received: 28 June 2018
Accepted: 29 July 2018

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 polycyclic aromatic hydrocarbons. 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.