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We report new correlations between ratios of band intensities of the 15-20{\mu}m emission bands of polycyclic aromatic hydrocarbons (PAHs) in a sample offifty-seven sources observed with Spitzer/IRS. This sample includes LargeMagellanic Cloud point sources from the SAGE-Spec survey, nearby galaxies fromthe SINGS survey, two Galactic ISM cirrus sources and the spectral maps of theGalactic reflection nebulae NGC 2023 and NGC 7023. We find that the 16.4, 17.4and 17.8 {\mu}m band intensities are inter-correlated in all environments. InNGC 2023 and NGC 7023 these bands also correlate with the 11.0 and 12.7 {\mu}mband intensities. The 15.8 {\mu}m band correlates only with the 15-20 {\mu}mplateau and the 11.2 {\mu}m emission. We examine the spatial morphology ofthese bands and introduce radial cuts. We find that these bands can bespatially organized into three sets: the 12.7, 16.4 and 17.8 {\mu}m bands; the11.2, 15.8 {\mu}m bands and the 15-18 {\mu}m plateau; and the 11.0 and 17.4{\mu}m bands. We also find that the spatial distribution of the 12.7, 16.4 and17.8 {\mu}m bands can be reconstructed by averaging the spatial distributionsof the cationic 11.0 {\mu}m and neutral 11.2 {\mu}m bands. We conclude that the17.4 {\mu}m band is dominated by cations, the 15.8 {\mu}m band by neutralspecies, and the 12.7, 16.4 and 17.8 {\mu}m bands by a combination of the two.These results highlight the importance of PAH ionization for spatiallydifferentiating sub-populations by their 15-20 {\mu}m emission variability.

Probing the Ionization States of Polycyclic Aromatic Hydrocarbons via the 15-20 {\mu}m Emission Bands