The temperature and density dependence of spin quantum beats of electrons is measured by time-resolved photoluminescence spectroscopy and yields the electron Landé g factor in bulk GaAs, InP, and CdTe. In GaAs the g factor increases linearly from Ϫ0.44 at 4 K to Ϫ0.30 at 280 K; in InP the g factor is 1.20 at 4 K, exhibiting a very small temperature dependence up to 160 K, and in CdTe the g factor follows between Tϭ4 K and 240 K the empirical equation gϭϪ1.653ϩ4ϫ10 Ϫ4 Tϩ2.8ϫ10 Ϫ6 T 2 . In GaAs we demonstrate the suppression of spin quantum beats due to Fermi blocking in a degenerate electron gas and measure an increase of the GaAs g factor from Ϫ0.44 at densities below 1ϫ10 16 cm Ϫ3 to Ϫ0.33 at 10 17 cm Ϫ3 .

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In our investigations of the photoluminescence in poly-͑para-phenylene͒-type ladder-polymer films a highly nonlinear dependence of the emission spectra is observable at elevated excitation densities. A spectral redistribution results in the hyperlinear increase of emission efficiency in the blue region, as the vibronic 0-1 transition (ϭ492 nm) becomes the most predominant radiative relaxation. The detailed study of temporal decay and spectral evolution of luminescence provided by streak camera measurements performed in the temperature range Tϭ20-300 K reveals the spectrum being composed by the contributions of two different emitting species, namely, S 1 -singlet states and aggregates. The material is found to be photochemically stable and luminescence does not seem to be limited by saturation effects even under high-excitation conditions, which is essential for laser application. ͓S0163-1829͑97͒03427-9͔ PICOSECOND SPECTROSCOPY AND HYPERLINEAR . . .