Mechanism of transient photoinduced absorption in conjugated polymers

We examine theoretically two possible origins of transient photoinduced absorption of conjugated polymers: absorption from an exciton-polaron with Bu symmetry and from the optically forbidden 2Ag state. Exact calculations for the PariserParr-Pople (PPP) model demonstrate that absorption spectra from 2Ag and from 1Bu are quite similar when their energies are nearly degenerate. We discuss the effects of lattice relaxation on spectral features and the natures of transitions in absorption from 1Bu within configuration-interaction calculations of the PPP model.