  We analyze resonances in the double photoionization of a variety of aromatic
hydrocarbons. The resonances reflect the breakup of quasi-bound electron pairs.
The basic premise of this paper is that there is a direct connection between
the quasi-bound pairs and resonant peaks in the optical absorption that are
associated with doubly occupied sites on the perimeter and inside the perimeter
of the molecule. The optical absorption leading to the high-energy resonance
(approximately 40 eV), calculated from a many-site one-dimensional Hubbard
model, has a peak at U, the electrostatic interaction energy for two electrons
with antiparallel spins on the same carbon atom. In the model, there are also
two satellites whose separation from the main resonance is approximately +/-10
eV suggesting that unresolved satellite structure may be contributing to the
linewidth of the resonant peak. The low energy resonances (approximately 10 eV)
involve carbon atoms located inside the perimeter, a configuration present only
in pyrene and coronene (among the hydrocarbons studied). In the case of pyrene,
which has two carbon atoms inside the perimeter, we employ a two-site Hubbard
model to characterize the absorption leading to the quasi-bound state. A brief
analysis of the double photoionization resonance of the heterocyclic inorganic
molecule 1,3,5-triazine presented. We also discuss recent results for the
double photoionization of the cyclic inorganic molecule tribromoborazine and
the organic molecules furan, pyrrole, selenophene, and thiophene where the 2+
ion concentration varies linearly with the difference between the photon energy
and the threshold energy. A theory for the linear behavior is outlined.
