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We present 5-20 micron spectral maps of the reflection nebula NGC2023obtained with the Infrared Spectrograph SL and SH modes on board the SpitzerSpace Telescope which reveal emission from polycyclic aromatic hydrocarbons(PAHs), C60, and H2 superposed on a dust continuum. We show that several PAHemission bands correlate with each other and exhibit distinct spatialdistributions revealing a spatial sequence with distance from the illuminatingstar. We explore the distinct morphology of the 6.2, 7.7 and 8.6 micron PAHbands and find that at least two spatially distinct components contribute tothe 7--9 micron PAH emission in NGC2023. We report that the PAH features behaveindependently of the underlying plateaus. We present spectra of compact ovalPAHs ranging in size from C_66 to C_210, determined computationally usingdensity functional theory, and investigate trends in the band positions andrelative intensities as a function of PAH size, charge and geometry. Based onthe NASA Ames PAH database, we discuss the 7--9 micron components in terms ofband assignments and relative intensities. We assign the plateau emission tovery small grains with possible contributions from PAH clusters and identifycomponents in the 7--9 micron emission that likely originates in thesestructures. Based on the assignments and the observed spatial sequence, wediscuss the photochemical evolution of the interstellar PAH family as they aremore and more exposed to the radiation field of the central star in theevaporative flows associated with the PDRs in NGC2023.

The PAH emission characteristics of the reflection nebula NGC2023