  In a recently performed experiment Korovyanko et al [Phys. Rev. B 67, 035114
(2003)] have measured the photo-induced absorption (PA) spectrum of
phenyl-disubstituted polyacetylenes (PDPA) from 1$B_{u}$ and 2$A_{g}$ excited
states. In 1$B_{u}$ PA spectrum they identified two main features namely PA1
and PA2. While in the 2$A_{g}$ spectrum they identified only one feature called
PA$_{g}$. In this paper we present a theoretical study of 1$B_{u}$ and 2$A_{g}$
PA spectra of oligo-PDPA's using correlated-electron Pariser-Parr-Pople (P-P-P)
model and various configuration interaction (CI) methodologies. We compare the
calculated spectra with the experiments, as well as with the calculated spectra
of polyenes of the same conjugation lengths. Calculated spectra are in good
agreement with the experiments. Based upon our calculations, we identify PA1 as
the m$A_{g}$ state and PA$_{g}$ as the $nB_{u}$ state of the polymer. Regarding
the PA2 feature, we present our speculations. Additionally, it is argued that
the nature of excited states contributing to the $2A_{g}$-PA spectra of
oligo-PDPA's is qualitatively different from those contributing to the spectra
of polyenes.
