ABSTRACT
The spectroscopic information from dust, however, is poor. Moreover, the emission is restricted to infrared wavelengths. The smallest grains, polycyclic aromatic hydrocarbon (PAHs), which show very characteristic bands are a fortunate exception. The first observation of PAHs, without proper identification, dates back to the early 1970s when in the near and mid infrared a number of hitherto unknown features were found. Although their overall spectral signature was subsequently observed in very diverse objects (HII regions, reflection nebulae, planetary nebulae, cirrus clouds, evolved stars, star burst nuclei and other external galaxies), their proper identification took place only in the mid 1980s. The PAH is built up of three benzene rings; all bonds at its edges are saturated by hydrogen atoms. The vibrational modes can be computed if one knows the force field (force constants) or the potential in the molecule.
