  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 of
fifty-seven sources observed with Spitzer/IRS. This sample includes Large
Magellanic Cloud point sources from the SAGE-Spec survey, nearby galaxies from
the SINGS survey, two Galactic ISM cirrus sources and the spectral maps of the
Galactic reflection nebulae NGC 2023 and NGC 7023. We find that the 16.4, 17.4
and 17.8 {\mu}m band intensities are inter-correlated in all environments. In
NGC 2023 and NGC 7023 these bands also correlate with the 11.0 and 12.7 {\mu}m
band intensities. The 15.8 {\mu}m band correlates only with the 15-20 {\mu}m
plateau and the 11.2 {\mu}m emission. We examine the spatial morphology of
these bands and introduce radial cuts. We find that these bands can be
spatially organized into three sets: the 12.7, 16.4 and 17.8 {\mu}m bands; the
11.2, 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 and
17.8 {\mu}m bands can be reconstructed by averaging the spatial distributions
of the cationic 11.0 {\mu}m and neutral 11.2 {\mu}m bands. We conclude that the
17.4 {\mu}m band is dominated by cations, the 15.8 {\mu}m band by neutral
species, 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 spatially
differentiating sub-populations by their 15-20 {\mu}m emission variability.

Astrophysics of Galaxies