Photoluminescence (PL), selective photoluminescence, and photoluminescence excitation (PLE) spectra have been measured on high-purity ZnSe single crystals at 4.2 K. Zn-dip treatment at 973 K for a week makes the I1d line disappear and the effective temperature of free-exciton gas decrease from 8.4 to 5.6 K through an increase in the lifetime of the free excitons. Accurate agreement is found for the transition energies of donor-bound excitons and their excited states with the values reported by Dean et al. Accurate values are estimated for the effective-mass donor binding energy and the static dielectric constant. The notable spectral feature in the PL spectra is that the emission intensities of the I3 lines are higher than those of the I2 lines. The origin of I3, the recombination of the excitons bound to ionized donors, is considered to be unreliable, judging from the donor-concentration dependence of the intensity ratio between the I2 and I3 emission lines and the spectral change under a cw dye-laser excitation with the energy below the band gap. PLE spectra show that the peaks related to excited states of I2 exist in the PLE spectra for I3 in addition to the other peaks related to the excited states of I3, and that the spectra for I3s is similar to that for I3w.
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