  We discuss the Aromatic Infrared Band (AIB) profiles observed by ISO-SWS
towards a number of bright interstellar regions where dense molecular gas is
illuminated by stellar radiation. Our sample spans a broad range of excitation
conditions (exciting radiation fields with effective temperature, Teff, ranging
from 23,000 to 45,000 K). The SWS spectra are decomposed coherently in our
sample into Lorentz profiles and a broadband continuum. We find that the
individual profiles of the main AIBs at 3.3, 6.2, 8.6 and 11.3 microns are well
represented with at most two lorentzians. Furthermore, we show that the
positions and widths of these AIBs are remarkably stable (within a few cm-1).
We then extract the profiles of individual AIBs from the data and compare them
to a model of Polycyclic Aromatic Hydrocarbon (PAH) cation emission which
includes the temperature dependence of the AIB profiles. The present similarity
of the AIB profiles requires that the PAH temperature distribution remains
roughly the same whatever the radiation field hardness. Deriving the
temperature distribution of interstellar PAHs, we show that its hot tail, which
controls the AIB spectrum, sensitively depends on Nmin (the number of C-atoms
in the smallest PAH) and Teff. Comparing the observed profiles of the
individual AIBs to our model results, we can match most of the AIB profiles if
Nmin is increased with Teff. We then discuss our results in the broader context
of ISO observations of fainter interstellar regions where PAHs are expected to
be in neutral form.
