  We present 10-15 micron spectra of a sample of H II regions, YSOs and evolved
stars that show strong unidentified infrared emission features, obtained with
the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of
emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 micron.
These features are observed to vary considerably in relative strength to
each-other from source to source. In particular, the 10-15 micron spectra of
the evolved stars are dominated by the 11.2 micron band while for H II regions
the 12.7 is typically as strong as the 11.2 micron band. Analysing the ISO data
we find a good correlation between the 11.2 micron band and the 3.3 micron
band, and between the 12.7 micron and the 6.2 micron band. There is also a
correlation between the ratio of the UIR bands to the total dust emission and
the 12.7 over 11.2 micron ratio. Bands in the 10-15 micron spectral region are
due to CH out-of-plane (OOP) bending modes of polycyclic aromatic hydrocarbons
(PAHs). We summarise existing laboratory data and theoretical quantum chemical
calculations of these modes for neutral and cationic PAHs. Due to mode
coupling, the exact peak position of these bands depends on the number of
adjacent CH groups and hence the observed interstellar 10-15 micron spectra can
be used to determine the molecular structure of the interstellar PAHs emitting
in the different regions. We conclude that evolved stars predominantly inject
compact ~100-200 C-atom PAHs into the ISM where they are subsequently
processed, resulting in more open and uneven PAH structures.
