Transient absorption anisotropy of a polythiophene polymer in a thin film was studied on a femtosecond time scale. The anisotropy has a non-exponential decay on the sub-picosecond time scale, with a fastest component characterized by an $\sim 40\mathrm{fs}$ time constant. To simulate the anisotropy decay an incoherent energy migration model has been used. Comparison between the simulated and experimental kinetics enabled us to estimate the nearest-neighbor pair wise hopping time $({\tau }_h=1\mathrm{}\pm 0.1\mathrm{}\mathrm{ps}),$ the fraction of the interchain aggregates $(\sim 10\%)$ and the structural disorder of the polymer. The initial $\sim 30\mathrm{fs}$ anisotropy decay does not originate from incoherent hopping energy transfer but from some other relaxation among electronic excited states within a spectroscopic unit.

• Received 14 October 2002

DOI:https://doi.org/10.1103/PhysRevB.67.205207