Effects of structural and chemical disorders on the visible/UV spectra
  of carbonaceous interstellar grains
  The recent spectacular progress in the experimental and theoretical
understanding of graphene, the basic constituent of graphite, is applied here
to compute, from first principles, the UV extinction of nano-particles made of
stacks of graphene layers. The theory also covers cases where graphene is
affected by structural, chemical or orientation disorder, each disorder type
being quantitatively defined by a single parameter. The extinction bumps
carried by such model materials are found to have positions and widths falling
in the same range as the known astronomical 2175 \AA features: as the disorder
parameter increases, the bump width increases from 0.85 to 2.5 $\mu$m$^{-1}$,
while its peak position shifts from 4.65 to 4.75 $\mu$m$^{-1}$. Moderate
degrees of disorder are enough to cover the range of widths of the vast
majority of observed bumps (0.75 to 1.3 $\mu$m$^{-1}$). Higher degrees account
for outliers, also observed in the sky.
  The introduction of structural or chemical disorder amounts to changing the
initial $sp^{2}$ bondings into $sp^{3}$ or $sp^{1}$, so the optical properties
of the model material become similar to those of the more or less amorphous
carbon-rich materials studied in the laboratory: a-C, a-C:H, HAC, ACH, coals
etc. The present treatment thus bridges gaps between physically different model
materials.
