Anharmonic infrared spectra of cationic pyrene and superhydrogenated derivatives

Studying the anharmonicity in the infrared (IR) spectra of polycyclic aromatic hydrocarbons (PAHs) at elevated temperatures is important to understand the vibrational features and chemical properties of interstellar dust, especially in the James Webb Space Telescope (JWST) era. We take pyrene as an example PAH and investigate how different degrees of superhydrogenation affect the applicability of the harmonic approximation and the role of temperature in the IR spectra of PAHs. This is achieved by comparing the theoretical IR spectra generated by classical molecular dynamics (MD) simulations and the experimental IR spectra obtained via gas-phase action spectroscopy, which utilizes the infrared multiple photon dissociation. All simulations are accelerated by a machine learning interatomic potential, in order to reach first-principles accuracies while keeping computational costs low. We have found that the harmonic approximation with empirical scaling factors is able to reproduce experimental band profile of pristine and partially superhydrogenated pyrene cations. However, a MD-based anharmonic treatment is mandatory in the case of fully superhydrogenated pyrene cation for matching theory and experiment. In addition, band shifts and broadenings as the temperature increases are investigated in detail. These findings may aid in the interpretation of JWST observations on the variations in band positions and widths of interstellar dust.