The infrared emission bands seen in many astronomical sources are widely attributed to the family of molecules known as polycyclic aromatic hydrocarbons (PAHs). Modelling of the observed emission requires a detailed knowledge of numerous PAH vibrational spectra. Theoretical quantum chemistry calculations have proven a valuable tool in studying the PAH vibrational spectra, which so far have been evaluated using the harmonic oscillator approximation. Taking advantage of recent developments of quantum chemical software, in This Work we evaluate the anharmonic vibrational spectra of small PAHs, thus extending the theoretical infrared spectra of PAHs into the anharmonic oscillator approximation. The main hurdle to this extension is that the anharmonic treatment of PAH molecules of astrophysical interest (Nc&gt;50) is extremely computationally demanding. To circumvent this issue, simple rules were developed from a small subset of representative PAHs in order to predict anharmonic effects of PAHs as a family, including intensity sharing (due to resonances) and frequency shifts of the bands. These rules can easily be applied to existing theoretical harmonic spectra and will greatly improve the accuracy of the modelling of PAH astronomical features.

Tackling the theoretical anharmonic infrared spectra of polycyclic aromatic hydrocarbons

249th National Meeting (2015)

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