  In this paper, we report theoretical calculations of the photo-induced
absorption (PA) spectrum of naphthalene and anthracene, with the aim of
understanding those excited states, which are invisible in the linear optical
absorption. The excited state absorption spectra are computed from the
$1B_{2u}^{+}$ and the $1B_{3u}^{+}$ states, and a detailed analysis of the
many-body character of the states contributing to various peaks in the spectra
is presented. The calculations are performed using the Pariser-Parr-Pople (PPP)
Hamiltonian, along with the full configuration interaction (FCI) technique. The
role of Coulomb parameters used in the PPP Hamiltonian is examined by
considering standard Ohno parameters, as well as a screened set of parameters.
The results of our calculations are extensively compared with the experimental
data where available, and very good agreement has been obtained. Moreover, our
calculations predict the presence of high intensity features which, to the best
of our knowledge have not been explored earlier. We also present concrete
predictions on the polarization properties of the PA spectrum, which can be
verified in experiments performed on oriented samples.
