Picosecond-luminescence study of exciton formation dynamics in CdSe

We make a direct determination of the time constant for free-exciton formation in CdSe at 8 K by measuring the picosecond time-resolved luminescence at a longitudinal-optical-phonon-assisted Stokes sideband of the free exciton. This method allows us to obtain the time evolution of the total exciton population, in contrast to measurements of luminescence due to $\overrightarrow{K}=\overrightarrow{0}$ excitons. The free-exciton formation time constant is found to be 7-10 ps, independent of the exciting photon energy within a few LO-phonon energies above the band-gap energy at the carrier excitation density of ≈6×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The results suggest rapid carrier thermalization and energy relaxation in the continuum before exciton formation, rather than hot exciton formation by the geminate electron-hole pairs followed by a cascade emission of LO phonos by the excitons.