Theory of bi-excitons in /spl pi/-conjugated polymers

Summary form only given. Theoretical models of /spl pi/-conjugated polymers that emphasize the Coulomb electron-electron correlations predict that the lowest optidal excitation is to an exciton. We have extended our theoretical studies to an energy region that is much higher, to probe specifically the states that are reached by two-electron excitations from the ground state. Theoretical evidence for the biexciton, a bound state of two excitons that occurs below the two-electron continuum, is found. The lowest biexciton state is of even parity, can be reached by optical excitatioll from the IB/sub u/ exciton, and in principle also by two-photon excitation from the ground state. Interpretation of a recent picosecond photoinduced absorption experiment on polyparavinylene (PPV) in terms of the biexciton will be given. Very similar experimental results have been obtained also in pump-probe experiments with other conjugated polymers (polydiacety-enes, PTV, etc.) in the past. These results will be discussed within the context of our theory. Our earlier work on optical nonlinearity is /spl pi/-conjugated polymers already predicted stable biexcitons, and the present results therefore also confirm the validity of the mechanism of third order optical nonlinearity proposed by us.