On the visible continuum and lines in the Interstellar Extinction Curve
  This work purports to help understand the InterStellar Extinction Curve in
and near the visible range. In this range, crystalline materials are known to
be transparent, so amorphous dust is needed. Molecular modeling experiments are
used to compute the electronic spectra of various, relatively large, carbon and
silicate structures. Hardly any transition shows up beyond 0.4 mum when the
structure is in its ground state (the lowest, most stable state, usually
crystalline). This is no longer the case as soon as the structure is distorted
in any way. Examples of simulated distortions (or ``defects'') are: angular or
linear bond alteration, insertion of free radicals near the main structure,
dangling bonds; their cumulative effects lead to the amorphous state. It is
shown that, in this state, a structure bears a majority of weak transitions and
a minority of strong ones. As the structure grows in size, the former
ultimately form a weak continuum already detected experimentally, in the
visible, on amorphous carbons and silicates. The stronger transitions will
manage to emerge above the continuum, especially when they bunch together by
chance near the same wavelength. Parallels are drawn between several properties
of the computed continua and transitions and the observed continuum and Diffuse
Interstellar Bands.
