Fully anharmonic infrared cascade spectra of polycyclic aromatic hydrocarbons

TL;DR

This paper develops a comprehensive theoretical framework for modeling the fully anharmonic IR emission spectra of interstellar PAHs, accounting for complex vibrational interactions and temperature effects, to better match astronomical observations.

Contribution

It introduces a detailed anharmonic vibrational perturbation theory approach and a simplified scheme for calculating IR cascade spectra of PAHs, improving upon previous models.

Findings

Corrected previous assumptions about anharmonic correction factors.

Demonstrated the importance of Fermi resonances in IR spectra.

Provided a new method for transforming absorption spectra into emission spectra.

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.