  We study rectangular graphene flakes using mean field states as the basis for
a configuration interaction calculation, which allows us to analyze the low
lying electronic excited states including electron correlations beyond the mean
field level. We find that the lowest energy transition is polarized along the
long axis of the flake, but the charge distributions involved in these
transitions are invariably localized on the zigzag edges. We also investigate
the impact of both short and long range impurity potentials on the optical
properties of these systems. We predict that even a weak impurity localized at
a zigzag edge of the flake can have a significant -- and often dramatic --
effect on its optical properties. This is in contrast to impurities localized
at armchair edges or central regions of the flake, for which we predict almost
no change to the optical properties of the flake even with strong impurity
potentials.
