The rich 6 to 9 μm spectrum of interstellar PAHs

IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species. Here, we present 6-9 mum 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 mum feature shifts from 6.22 to 6.3 mum and clearly shows profile variations. 2) The 7.7 mum complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 mum. In some cases the main peak can apparently shift up to 8 mum. Two sources do not exhibit a 7.7 mum complex but instead show a broad emission feature at 8.22 mum. 3) The 8.6 mum 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 mum 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 mum feature peaking at similar to6.22 mum exhibit a 7.7 mum complex dominated by the 7.6 mum component. In contrast, sources with a 6.2 mum profile peaking longwards of 6.24 mum show a 7.7 mum complex with a dominant peak longwards of 7.7 mum and a 8.6 mum feature shifted toward the red. Furthermore, the observed 6-9 mum 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 mum profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 mum component and substituted/complexed PAHs representing the 6.2 mum component. The 7.6 mum component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 mum component remains an enigma. In addition, the exact identification of the 8.22 mum 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.