FEMTOSECOND PUMP-PROBE SPECTROSCOPY OF CONJUGATED POLYMERS - COHERENT AND SEQUENTIAL CONTRIBUTIONS

We report femtosecond pump-probe difference absorption experiments on polydiacetylene-para-toluene sulfonate. Theoretical analysis using a three-mode Brownian oscillator model for the nuclear dynamics reveals spectral diffusion in this conjugated polymer; this effect cannot be accounted for using the optical Bloch equations. Sequential (pump first) time ordering is dominant for resonant excitation, whereas experiments utilizing an off-resonant pump are dominated by nonsequential time orderings of the pump and the probe; the latter represent coherent (Raman-type) processes.