FUNDAMENTAL ABSORPTION IN SOLIDS AND THE WAVELENGTH DEPENDENCE OF INTERSTELLAR EXTINCTION

It is found that the wavelength dependence of interstellar extinction between the 0.44-mu-m band and 8-mu-m follows a Tauc-Urbach law characteristic of amorphous semiconductors. This result implies that the majority of extinction in this wavelength range arises predominantly from an amorphous dust component. A comparison between laboratory and interstellar data suggests that this dust is likely to be amorphous carbon. The Tauc-Urbach dependence permits the ratio of total to selective extinction, R(V), to be related to the optical properties of this dust component and provides a semiquantitative relation between R(V) and environmental conditions in the interstellar medium. It also supports an important connection between the physics of condensed matter and the optics of interstellar dust.