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We decompose the observed 7.7 $\mu$m polycyclic aromatic hydrocarbon (PAH)emission complexes in a large sample of over 7000 mid-infrared spectra of theinterstellar medium (ISM) using spectral cubes observed with the Spitzer/IRS-SLinstrument. In order to fit the 7.7 $\mu$m PAH emission complex we invoke fourGaussian components which are found to be very stable in terms of their peakpositions and widths across all of our spectra, and subsequently define adecomposition with fixed parameters which gives an acceptable fit for all thespectra. We see a strong environmental dependence on the inter-relationshipsbetween our band fluxes - in the HII regions all four components areinter-correlated, while in the reflection nebulae (RNe) the inner and outerpairs of bands correlate in the same manner as previously seen for NGC~2023. Weshow that this effect arises because the RNe maps are dominated by stronglyirradiated PDR emission, while the much larger HII region maps are dominated byemission from regions much more distant from the exciting stars, leading tosubtly different spectral behavior. Further investigation of this dichotomyreveals that the ratio of two of these components (centered at 7.6 and 7.8$\mu$m) is linearly related to the UV field intensity (log G$_0$). We find thatthis relationship does not hold for sources consisting of circumstellarmaterial, which are known to have variable 7.7 $\mu$m spectral profiles.

Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps II: A direct link between band profiles and the radiation field strength