Kinetics of Excitons in CdS at Temperature

New experimental results of photoluminescence of CdS at He temperatures suggest two alternative models for the fate of an exciton in CdS. The models, presented below, include exciton-exciton interaction and explain in a self-consistent way all known experimental facts. One model assumes that the low over-all luminescent efficiency of CdS at low temperatures is due to the existence of a large concentration of nonradiative traps; it predicts a rate constant of 3×${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ for the trapping of excitons at traps giving rise to bound exciton luminescence, and a rate constant of 3×${10}^{-13\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ for exciton-exciton interaction which gives rise to luminescence. The alternative model assumes that the over-all loss in excitation is caused by the process of formation of excitons from electron-hole pairs. It uses only known radiative traps, and predicts a rate constant for exciton-exciton interaction of 5×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ and a rate constant for trapping of excitons at radiative traps of 3×${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$.