  Herbig Ae/Be stars are a class of young pre-main sequence stellar objects of
intermediate mass and are known to have varying amounts of natal cloud material
still present in their direct vicinity. We characterise the IR emission bands,
due to fluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars and
link observed variations to spatial aspects of the mid-IR emission. We analysed
two PAH dominated spectra from a sample of 15 Herbig Ae/Be stars observed with
Spitzer and derive profiles of the major PAH bands. The shape and the measured
band characteristics show pronounced variations between the two Spitzer
spectra. Those variations parallel those found between three ISO spectra of
other, well-studied, Herbig Ae/Be stars. The derived profiles are compared to
those from a broad sample of sources. The Spitzer and ISO spectra exhibit
characteristics commonly interpreted respectively as interstellar matter-like
(ISM), non-ISM-like, or a combination of the two. We argue that the PAH
emission detected from the sources exhibiting a combination of ISM-like and
non-ISM-like characteristics indicates the presence of two dissimilar,
spatially separated, PAH families. As the shape of the individual PAH band
profiles reflects the composition of the PAH molecules involved, this
demonstrates that PAHs in subsequent, evolutionary linked stages of star
formation are different from those in the general ISM, implying active
chemistry. None of the detected PAH emission can be associated with the
(unresolved) disk and is thus associated with the circumstellar cloud. This
implies that chemical changes may already occur in the natal cloud and not
necessarily in the disk.
