Femtosecond pump-probe investigation on relaxation of photoexcitations and spectral narrowing of photoluminescence for poly(para-phenylenevinylene)

Photoinduced absorption spectra and their temporal profiles of poly(para-phenylenevinylene) thin film exhibit a strong correlation with an appearance of spectrally narrowed emission bands. The dependence of transient absorption signals on excitation and probe power denotes much lower saturation intensity in photoinduced absorption than that in stimulated emission arising from the identical photoexcitations, singlet excitons. We interpreted the peculiar emission bands in terms of superradiance or superfluorescence with a time constant of around 1 ps rather than amplification of spontaneous emission from uncorrelated emitters.