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 illumination by the red Light from a He-Ne laser on the crystals pre-irradiated with X-ray. The temperature dependence of the BG emission intensity shows rather complicated behavior. As raising temperature, a prominent hump appears around 85 K, which is attributed to thermo-luminescence resulting from recombination of thermally activated holes with STEL centers. The intensity of the BG emission decreases at temperatures above 140 K, corresponding to thermal instability of self-trapped electron (STEL) centers. Time dependence of the luminescence after pulsed light excitation comprises two stages, namely, rather quick decay in the mu s region and long afterglow up to several tens ms. Two channels are proposed for the BG emission; one is the direct recombination process between the STEL and the hole center, the other radiative annihilation of the STE of the [(Pb-2)(3+)-STEL+hole] type.