BIEXCITONS IN PI-CONJUGATED OLIGOMERS - INTENSITY-DEPENDENT FEMTOSECOND TRANSIENT-ABSORPTION STUDY

We report femtosecond transient-absorption (TA) studies of a five-ring oligomer of poly(para-phenylene vinylene) prepared in two different f orms: solid-state films and dilute solutions. At high pump fluences, i n both types of samples. we observe generation of two-exciton states, which art: detected by the evolution of TA spectra and dynamics with i ncreasing pump intensity. In solutions, double excitation of molecules results in the formation of stable biexcitons with enhanced oscillato r strength, leading to an increased efficiency of the radiative decay and a superlinear pump dependence of the stimulated emission. In solid -state samples, the two-exciton states are unstable and decay on the s ubpicosecond time scale due to ultrafast charge transfer, accompanied by generation of interchain excitons. [S0163-1829(98)03336-0].