The vis/UV spectrum of coals and the interstellar extinction curve

Photoreflectance measurements followed by a Kramers-Kronig analysis have yielded the dielectric constants and refractive indices of polycrystalline graphite and two high-ranking coals. All three display pi and sigma+pi features which change regularly with the extent of graphitization. These results were confirmed by electron energy loss spectroscopy. Comparison of the deduced Q(ext)s with the IS extinction curve shows that anthracite is the closest fit to the 2175-A feature, while the carrier of the underlying continuum must be similar to low-rank, poorly graphitized coals. About 1/3 of the cosmic carbon is involved in the feature, if in the form of anthracite.