Fully anharmonic infrared cascade spectra of polycyclic aromatic hydrocarbons

Abstract

The infrared (IR) emission of polycyclic aromatic hydrocarbons (PAHs) permeates our universe; astronomers have detected the IR signatures of PAHs around many interstellar objects. The IR emission of interstellar PAHs differs from their emission as seen under conditions on Earth, as they emit through a collisionless cascade down through their excited vibrational states from high internal energies. The difficulty in reproducing interstellar conditions in the laboratory results in a reliance on theoretical techniques. However, the size and complexity of PAHs requires careful consideration when producing the theoretical spectra. In this work we outline the theoretical methods necessary to lead to a fully theoretical IR cascade spectra of PAHs including: an anharmonic second order vibrational perturbation theory (VPT2) treatment; the inclusion of Fermi resonances through polyads; and the calculation of anharmonic temperature band shifts and broadenings (including resonances) through a Wang--Landau approach. We also suggest a simplified scheme to calculate vibrational emission spectra that retains the essential characteristics of the full IR cascade treatment and can directly transform low temperature absorption spectra in IR cascade spectra. Additionally we show that past astronomical models were in error in assuming a 15 cm${}^{-1}$ correction was needed to account for anharmonic emission effects.

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• Anharmonicity and the infrared emission spectrum of highly excited PAHs
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• The High-Resolution Far- to Near-Infrared Anharmonic Absorption Spectra of Cyano-Substituted Polycyclic Aromatic Hydrocarbons from 300-6200 cm${}^{-1}$
• The infrared bands of polycyclic aromatic hydrocarbons in the 1.6-1.7 μm wavelength region
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• The 3.3 $\mu$m Infrared Emission Feature: Observational and Laboratory Constraints on Its Carrier
• SPHEREx: Aromatics, Aliphatics and PAH Size across the Iris Nebula
• Aromatic Species in the Molecular Universe
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• N-Mode Quantized Anharmonic Vibronic Hamiltonians for Matrix Product State Dynamics
• The Influence of Aliphatic Components on the Aromatic Emission Characteristics of Polycyclic Aromatic Hydrocarbons