Nature of the Self-Activated Blue Luminescence Center in Cubic ZnS: Cl Single Crystals

Over-all optical properties of cubic ZnS: Cl single crystals were studied in order to clarify the nature of the self-activated luminescence center. The characteristic polarizations found both in fluorescence and in thermoluminescence provide the first information about the optical symmetry inherent in the luminescence center in sulfide crystals. From details of the observed azimuthal dependences of the polarization, it is concluded that the luminescence is due to the $\sigma$ electric-dipole oriented along the tetrahedral bond direction in the cubic lattice of ZnS, and direct evidence is given for the identification of the luminescence center with the $A$ center responsible for the electron-spin resonance absorption. There were found two excited states in the polarization spectra, the lower one being located at about 3.42 eV (363 mμ) and the other at about 3.76 eV (330 mμ) above the ground state. According to the observed polarization rules for the optical transitions between these states, the ground state is assigned to an $A_1$ state, while the emitting state and the second excited state, respectively, are assigned to $E$ and $A_1$ states belonging to the $C_{3v}$ symmetry center, the luminescence being due to the $E\rightharpoonup A_1$ transition. Physical pictures of these energy levels are given by simple molecular-orbital treatment on the basis of the model of a ${\mathrm{Zn}}^{2+}$ ion vacancy associated with a substitutional ${\mathrm{Cl}}^{-}$ ion, as proposed by Prener and Williams, for the luminescence center. The effects of the stacking faults and of possible Jahn-Teller distortion expected in the excited center are also discussed.