  Here, we present 6-9 um spectra of a sample of RNe, HII regions, YSOs,
evolved stars and galaxies that show strong unidentified infrared bands,
obtained with the ISO-SWS. The IR emission features show pronounced variations.
1) The 6.2 um feature shifts and clearly shows profile variations. 2) The 7.7
um complex is comprised of at least two subpeaks peaking at 7.6 and one
longwards of 7.7 um. In some cases the main peak can apparently shift up to 8
um. Two sources do not exhibit a 7.7 um complex but instead show a broad
emission feature at 8.22 um. 3) The 8.6 um 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 um 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. Furthermore, the observed
6--9 um spectrum depends on the type of object. We attribute the observed 6.2
um profile and peak position to the combined effect of a PAH family and
anharmonicity with pure PAHs representing the 6.3 um component and
substituted/complexed PAHs representing the 6.2 um component. The 7.6 um
component is well reproduced by both pure and substituted/complexed PAHs but
the 7.8 um component remains an enigma. In addition, the exact identification
of the 8.22 um 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.
