We propose to study the molecular symmetries of aromatic molecules in
 the interstellar medium and determine how these symmetries change as
 interstellar carbon evolves from the free molecular phase to
 incorporation into dust particles.  These symmetries will be determined
 by searching for the characteristic infrared emission features produced
 by the C-H bending mode vibrations of specific polycyclic aromatic
 hydrocarbon (PAH) molecules and various forms of amorphous carbon in
 carbon-rich objects which span the evolutionary range from late carbon
 stars to molecular clouds.  We propose to achieve this goal by measuring
 the 10.7-15.4 micron spectra of these objects.  This spectral range
 spans the region where almost all PAH C-H out-of-plane bending modes
 fall.  We have carried out laboratory studies that show that the
 positions of these bands are extremely sensitive to the molecular
 symmetry of the PAH and therefore promise to provide one of the best
 means for identifying specific PAHs in the interstellar medium.
 Identifying specific PAHs in the overall population and determining how
 this population evolves in time will provide direct insight into the
 chemical processes that affect PAHs and the physical processes
 associated with dust nucleation and growth in circumstellar shells and
 the interstellar medium.  We propose to obtain spectra with ISO of
 sufficient quality to detect the many additional emission features which
 must be present longward of the well known 11.3 micron emission feature
 already observed from these objects.  With our lab data we will be able
 to interpret these spectra in terms of which specific PAH molecules and
 amorphous carbon structures are present in these objects and determine
 their abundances.  With data of the quality expected from ISO, it will
 finally be possible to probe directly into the process of carbon
 nucleation and growth, gaining insight into the evolution of carbon from
 its molecular form to larger amorphous particles, and understand which
 molecular species are associated with each type of object.