Investigation on relaxation dynamics of photoexcitations and spectral narrowing of photoluminescence in PPV films

Photoinduced absorption spectral feature as well as their temporal profiles on PPV thin film in femtosecond time regime exhibit a strong pump power density dependence. The increase in pump power density also results in the spectrally narrowed emission (SNE) band superimposed to the well-known photoluminescence from PPV. The dependencies on both pump- and probe densities denote much lower saturation intensity in photoinduced absorption as compared with that in stimulated emission from the identical primary photoexcitations, the singlet excitons. We have also discussed the nature of SNE, which has a correlation with an appearance of ultrafast decay component (τ1 ps) in temporal decay profile of the photoinduced absorption signal.