Anharmonic infrared spectra of cationic pyrene and superhydrogenated derivatives

Abstract

Studying the anharmonicity in the infrared (IR) spectra of polycyclic aromatic hydrocarbons (PAHs) at elevated temperatures is important to understand 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 affects the applicability of the harmonic approximation and the role of temperature in IR spectra of PAHs. This is achieved by comparing theoretical IR spectra generated by classical molecular dynamics (MD) simulations and experimental IR spectra obtained via gas-phase action spectroscopy which utilizes the Infrared Multiple Photon Dissociation (IRMPD). All simulations are accelerated by a machine learning interatomic potential, in order to reach first principle accuracies while keeping low computational costs. 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. Those findings may aid in the interpretation of JWST observations on the variations in band positions and widths of interstellar dust.

Figures (6)

• Figure 1: Top and side views of PAH cations: (a) pyrene (C$_{16}$H$_{10}^+$), (b) 4,5,9,10-tetrahydropyrene (C$_{16}$H$_{14}^+$, THP), (c) 1,2,3,6,7,8-hexahydropyrene (C$_{16}$H$_{16}^+$, HHP) and (d) perhydropyrene (C$_{16}$H$_{26}^+$, PHP). Color codes are: gray (aromatic carbon), light yellow (aliphatic carbon), white (hydrogen).
• Figure 2: IR spectra of pyrene, THP, HHP and PHP radical cations within the harmonic approximation. Solid lines are DFT results while colored dashed lines are generated with MLIP.
• Figure 3: Comparisons between theoretical and experimental IR spectra of (a) pyrene; (b) THP; (c) HHP and (d) PHP. Red lines are obtained from MLMD simulations, while blue lines are from harmonic calculations using the MLIP and a empirical scaling factor of 0.9671 is applied on harmonic frequencies. Experimental IR spectra are shown in gray lines. Each spectrum has been normalized to its maximum peak intensity.
• Figure 4: Temperature-dependent IR spectra of the pyrene cation from MLMD simulations. (a) Frequency ranges from 500 to 1900 cm$^{-1}$ (b) the C-H out-of-plane bending mode in the frequency region 820 - 920 cm$^{-1}$ (c) the C-H in-plane bending mode in the frequency region 1200 - 1300 cm$^{-1}$ (d) the C-C stretching mode in the frequency region 1500 - 1600 cm$^{-1}$
• Figure 5: Quantitative analysis of the role of temperature for three modes in the pyrene cation. (a) band position in cm$^{-1}$ (b) band full width at half maximum (FWHM) in cm$^{-1}$