Large-scale correlated study of excited state absorptions in naphthalene and anthracene

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.