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$\mu$m, 5.5$\mu$m and 10.6$\mu$m; at longer wavelengths, there is a forest
of emission features in the 16--30$\mu$m range that appears as a structured
continuum, but with a clear peak centered around 19$\mu$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$\mu$m in all
positions except that closest to the central star. We also find evidence for a
weak 19$\mu$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.
