We present 15 – 20 \mu m long-slit spectra , from the Infrared Spectrograph ( IRS ) on Spitzer , of NGC 7023 .
We observe recently-discovered interstellar emission features , at 15.9 , 16.4 , 17.0 , 17.4 , 17.8 , and 18.9 \mu m , throughout the reflection nebula .
The 16.4 \mu m emission feature peaks near the photodissociation front northwest of the star , as do the aromatic emission features ( AEFs ) at 3.3 , 6.2 and 11.3 \mu m. The 16.4 \mu m emission feature is thus likely related to the AEFs and radiates by non-equilibrium emission .
The new 18.9 \mu m emission feature , by contrast , decreases monotonically with stellar distance .
We consider candidate species for the 18.9 \mu m feature , including polycyclic aromatic hydrocarbons , fullerenes , and diamonds .
We describe future laboratory and observational research needed to identify the 18.9 \mu m feature carrier .