Figure 2.

Astronomical spectra of the infrared vibrational modes (at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm) of PAH molecules in three objects that represent the different peak positions and relative intensities observed in various galactic and extragalactic regions (see Peeters et al. 2002). Classes A, B and C represent the three different astronomical environments; (A) ISM, reflection nebulae, HII regions; (B) a few post-AGB and Herbig Ae/Be stars and most Planetary Nebulae; (C) a few peculiar post-AGB stars.