Interpreting interstellar observations of polycyclic aromatic hydrocarbons (PAHs) relies heavily on theoretical infrared spectra. The majority of studies to date make use of scaled harmonic frequencies for band positions, the double harmonic approximation for intensities, and ad hoc band shifts of 15 cm<sup>-1</sup> to account for anharmonicities. Low-temperature high-resolution gas-phase experiments have shown recently that this harmonic treatment cannot reproduce reliably the experimental results, especially for the C-H stretching modes around 3.3 μm. We have shown recently that neglecting strong Fermi resonances causes the discrepancy between theory and experiment. In order to account for these Fermi resonances anharmonicities need to be introduced into the theory. Specifically, a second order vibrational perturbation approach, which takes into account large numbers of simultaneous resonances (or polyads), is required. This approach has been shown to reproduce successfully the low-temperature spectra of many representative PAHs. However, interstellar PAHs are vibrationally hot. Fortunately, anharmonic calculations can also be used to produce spectra at any given temperature where band shifts and broadenings due to the temperature effects are accounted for naturally. We will show how this approach can be applied to interstellar PAHs with specific attention to the 3.3 μm feature seen in the emission spectra of interstellar PAHs.

Temperature dependent 3.3 μm spectra of PAHs: An anharmonic theoretical approach

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.