Top panel: mid-IR spectra of astronomical sources illustrating examples of type~D (IRAS 05110–6616; black), type C (IRAS 13416-6243; blue), type~B (HD 44179; red), and type A (IRAS 23133+6050; orange), all normalized to the maximum intensity of the 7–8 μm complex and with continuum subtracted. The type D spectrum was recorded by Spitzer/IRS (Werner et al. 2004; Houck et al. 2004), while the others are from ISO/SWS (Kessler et al. 1996; de Graauw et al. 1996), as calibrated by Sloan et al. (2003). Bottom panel: IRMPD spectra of pery^•+ (orange), pero^•+ (red), ova^•+ (blue), and iso^•+ (black) in the 6–10 μm spectral range.