Characterizing the infrared spectra of small, neutral, fully dehydrogenated PAHs

We present the results of a computational study to investigate the infrared spectroscopic properties of a large number of polycyclic aromatic hydrocarbon (PAH) molecules and their fully dehydrogenated counterparts. We constructed a database of fully optimized geometries for PAHs that is complete for eight or fewer fused benzene rings, thus containing 1550 PAHs and 805 fully dehydrogenated aromatics. A large fraction of the species in our database have clearly non-planar or curved geometries. For each species, we determined the frequencies and intensities of their normal modes using density functional theory calculations. Whereas most PAH spectra are fairly similar, the spectra of fully dehydrogenated aromatics are much more diverse. Nevertheless, these fully dehydrogenated species show characteristic emission features at 5.2$μ$m, 5.5$μ$m and 10.6$μ$m; at longer wavelengths, there is a forest of emission features in the 16--30$μ$m range that appears as a structured continuum, but with a clear peak centered around 19$μ$m. We searched for these features in Spitzer-IRS spectra of various positions in the reflection nebula NGC 7023. We find a weak emission feature at 10.68$μ$m in all positions except that closest to the central star. We also find evidence for a weak 19$μ$m feature at all positions that is not likely due to C$_{60}$. We interpret these features as tentative evidence for the presence of a small population of fully dehydrogenated PAHs, and discuss our results in the framework of PAH photolysis and the formation of fullerenes.

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