We propose to probe the evolution of interstellar carbon from the
 molecular to the particle phase by searching for the infrared emission
 signatures 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 5-9 micron spectra of these
 objects, the spectral region which is most diagnostic of these materials
 and the region in which they emit most strongly.  This will provide
 direct insight into the processes of carbon nucleation and growth in
 circumstellar shells and the interstellar medium.  This is the opportune
 time for this project.  We propose to obtain spectra with ISO of
 sufficient quality to detect the spectral substructure which must be
 present superimposed on the larger spectral features already observed in
 emission from these objects.  With our lab data we will be able to
 interpret these spectra in terms of which molecules and amorphous carbon
 structures are actually 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 species are
 associated with each type of object.