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The dynamics of exciton-polariton recombination in CdS are investigated by measuring time-resolved. luminescence spectra between 1.5 and 80 K. Decay times of free and bound excitons in Elc and E~~c polarization are determined for various types of crystals using low-excitation densities. The experiments prove the existence of a bottleneck for the relaxation and establish its location. Transverse excitons above the "knee" of the dispersion curve are found to be in thermal equilibrium with the lattice at temperatures higher than 30 K. At low lattice temperatures, excitons above the "knee" are in partial quasiequilibrium shortly after pulse excitation. The effective temperature of this exciton distribution exceeds that of the lattice and decreases rapidly within the observed exciton lifetime of 2.5 -3 nsec.

The undulation spectrum occurs in the luminescence of Gap when it is doped with the isoelectronic trap, nitrogen, and an acceptor -in the present case Zn. A broad emission band is observed which extends about 20 meV below the nitrogen bound-exciton energy EA and contains regular undulations. We show that the luminescence originates from the decay of excitons bound to N-Zn pairs of up to 50-A separation, and that the undulatory behavior is a result of averaging the numbers of equivalent sites in the pair-separation shells.The N-Zn interaction is investigated by time-resolved resonant excitation and temperature-dependent measurements. The 1.6-K decay lifetime of the luminescence decreases sharply with luminescence energy from -1 p.sec at E" to about 50 nsec at the low-energy limit of the undulation spectrum. The splitting of the exciton energy levels increases up to 5 meV as the N-Zn separation decreases and the dominant interaction is thought to be an internal Stark effect. Spatial energy transfer by tunneling of excitons is important and determines the temperature-dependent nonradiative recombination.