THE EFFECT OF DISORDER ON OPTICAL-SPECTRA OF CONJUGATED POLYMERS

Using the semi-empirical parametrization of the Pariser-Parr-Pople model we determine the excitonic states in conjugated polymers. The employed parameters correspond to a moderate strength of the long-range Coulomb interaction. In order to take the disorder into account, the nearest-neighbour transfer matrix elements are chosen according to a random distribution. Averaging the results over a large number of chains of 100 sites, which are known to be sufficiently long so that finite-size effects become negligible, we obtain the linear and third-order nonlinear optical susceptibilities. With increasing disorder the linear absorption, the two-photon absorption and the third-harmonic generation spectra display an increasing shift, broadening and growing asymmetry of the lines. Contrastingly, the life-time broadening yields neither a shift nor an asymmetry. The comparison with experimental spectra shows that the consideration of disorder effects improves the agreement between theory and experimental data.