On the visible continuum and bands in the interstellar extinction curve II

Abstract

Any distortion of a chemical structure causes new features to appear in the absorption spectrum of the structure, especially in the visible and near UV (see Paper I). Chemical modeling, using molecular orbital theory, showed that the continuum resulting from the accumulation of weak new bands in that range, correctly mimics the continuum measured in the laboratory on pure synthesized silicates, in the transparency spectral range, as well as in the InterStellar extinction curve in the same range. The present paper explores in more detail the strong discrete bands that emerge from the continuum due to distortion. It is found that different types of structure (linear or compact, carbon- or silicon-bearing) have each a limited number of strong, characteristic, bands at different wavelengths. Distortion is only one instance of ``defects'' that enrich the vis/UV absorption spectrum; others are: vacancies and voids, substitutions, inclusions in interstices, impurities, dangling bonds. The accumulation of these result in an ``amorphous'' structural state. Several examples of known amorphous materials, both carbon- and silicon-bearing, that have been analyzed in the laboratory, and simulated theoretically, are described below, thus extending the scope of this work. A final section lists several fields of astrophysics that have used, or may use, amorphous models of dust.