The rich 6 to 9 $\vec\mu$m spectrum of interstellar PAHs

IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species. Here, we present 6–9 spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2 feature shifts from 6.22 to 6.3 and clearly shows profile variations. 2) The 7.7 complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 . In some cases the main peak can apparently shift up to 8 . Two sources do not exhibit a 7.7 complex but instead show a broad emission feature at 8.22 . 3) The 8.6 feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6 features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2 feature peaking at ~6.22 exhibit a 7.7 complex dominated by the 7.6 component. In contrast, sources with a 6.2 profile peaking longwards of 6.24 show a 7.7 complex with a dominant peak longwards of 7.7 and a 8.6 feature shifted toward the red. Furthermore, the observed 6–9 spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2 profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 component and substituted/complexed PAHs representing the 6.2 component. The 7.6 component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 component remains an enigma. In addition, the exact identification of the 8.22 feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.