Picosecond radiative decay of free and bound excitons and excitonic molecules in CdSe

Band-edge luminescence in photoexcited CdSe is known to show several interesting features due to radiative decay of free and bound excitons and excitonic molecules (biexcitons). In this paper, we report our experiments on the study of the time evolution of some of these processes in CdSe, using up-conversion luminescence spectroscopy with a time resolution of 2.5 ps. We obtain time-dependent luminescence spectra at two e$hyh pair excitation densities (estimated to be ${n}₀=8×{10}¹⁶and 8×{10}¹⁷$ ${cm}^{{{-}}3}$). The role of hot biexciton dynamics in determining the time evolution of luminescence is evident. At low density (${n}₀=8×{10}¹⁶$ ${cm}^{{{-}}3}$), the luminescence spectrum also reveals the time evolution of the bound exciton emission at long delays (>200 ps). In addition, we obtain time-resolved luminescence of the longitudinal-optical phonon-assisted Stokes sidebands of the free exciton. The exciton and biexciton lifetimes are deduced to be 600 and 10 ps, respectively, on the basis of a theoretical model describing their coupled dynamics.