  In this paper we present a computational study of linear optical absorption
in phenacene class of polyaromatic hydrocarbons. For the purpose, we have
employed a correlated-electron methodology based upon configuration-interaction
(CI) approach, and the Pariser-Parr-Pople (PPP) $\pi$-electron model
Hamiltonian. The molecules studied range from the smallest one with three
phenyl rings (phenanthrene) to the largest one with nine phenyl rings. These
structures can also be seen as finite-sized hydrogen-passivated armchair
graphene nanoribbons of increasing lengths. Our CI calculations reveal that the
electron-correlation effects have a profound influence not just on the peak
locations, but also on the relative intensity profile of the computed spectra.
We also compare our phenacene results with isomeric oligo-acenes, and find that
in all the cases former have a wider optical gap than the latter. Our results
are found to be in very good agreement with the experiments, where available.
