DECAY TIME STUDIES ON UV-LUMINESCENCE IN CDBR2-CDCL2 MIXED-CRYSTALS

The decay time measurements have been carried out on the self-trapped exciton (STE) luminescence in CdBr2, CdCl2 and CdCl2-CdBr2 mixed cryst als. The strong STE luminescence is observed at low temperatures below 50 K in the near ultra-violet region (UV-luminescence) having the pea k at 3.26 eV in CdBr2 and 3.76 eV in CdCl2. In the decay curve of the UV-luminescence appear two components, the fast one with the ns life t ime and the slow one with the mu s life time which are related to the spin-singlet and -triplet STE's, respectively. The time integrated int ensity ratio of the singlet to the triplet component is much smaller i n CdBr2 (0.064) than in CdCl2 (32) reflecting the difference in streng th of spin-orbit interaction between Br- and Cl-. In the mixed crystal s of CdCl2(1-x)-CdBr2(x), the value of the time integrated intensity d ecreases rapidly with small increase of the CdBr2 content, x, from the value in CdCl2 to that in CdBr2. This means that the STE state strong ly depends on the Br- content. The experimental results are explained by means of a structural model for the STE in cadmium halide crystals, namely, the excited states of [Cd(2+)X(6)(-)](4-) complex molecular i ons with D-3d symmetry.