LUMINESCENCE DUE TO DIMER TYPE SELF-TRAPPED EXCITONS IN LEAD HALIDES

Recombination luminescence in lead halides (PbCl2 and PbBr2) has been investigated by the VUV light excitation above the band gap energy. An intense blue-green (BG) emission band is observed at 2.62 eV both in PbCl2 and in PbBr2. The same BG emission is also observed under illumi nation by the red Light from a He-Ne laser on the crystals pre-irradia ted with X-ray. The temperature dependence of the BG emission intensit y shows rather complicated behavior. As raising temperature, a promine nt hump appears around 85 K, which is attributed to thermo-luminescenc e resulting from recombination of thermally activated holes with STEL centers. The intensity of the BG emission decreases at temperatures ab ove 140 K, corresponding to thermal instability of self-trapped electr on (STEL) centers. Time dependence of the luminescence after pulsed li ght excitation comprises two stages, namely, rather quick decay in the mu s region and long afterglow up to several tens ms. Two channels ar e proposed for the BG emission; one is the direct recombination proces s between the STEL and the hole center, the other radiative annihilati on of the STE of the [(Pb-2)(3+)-STEL+hole] type.