Model of transient photoinduced absorption with an exciton polaron in conjugated polymers

A theoretical description of a spin-singlet exciton polaron is given by incorporating both electron-electron and electron-lattice interactions to discuss transient photoinduced absorption in conjugated polymers. Absorption spectra with transitions from a relaxed exciton polaron state as well as from an unrelaxed Franck-Condon state to higher-lying excited states were calculated within the frozen-lattice approximation and the configuration interaction among single and double excitations for chains of up to 24 sites. Main absorption peaks are situated in the region of relatively low photon energies for both unrelaxed and relaxed lattices, although the energy levels responsible for the peaks are different, After the lattice relaxation, oscillator strength is more concentrated on a single peak.