We have calculated the two-photon absorption (TPA) spectrum in the Pariser-Parr-Pople model of conjugated polymers. Configuration interactions among all the single and double excitations are taken into account for relatively long chains. It is shown that the maximum of TPA decreases with increasing interaction strength or with increasing bond alternation. For moderate interaction strength and bond alternation, the overall TPA spectrum is characterized by two energy regions: the low-energy part due to mainly one-electron excitations (excitonic states), and the higher-energy part with contributions mainly from two-electron excited states (biexcitonic states). The high-energy TPA is about one order of magnitude stronger than the low-energy part. For stronger interaction or smaller bond alternation, the single and double excitations are fairly mixed, and the distinction between the excitonic and the biexcitonic regions is lost.

• Received 4 April 1994

DOI:https://doi.org/10.1103/PhysRevB.50.4306