Luminescence of PbCl 2 and PbBr 2 single crystals II. Luminescence and EPR of uv irradiated crystals
PbCl 2 and PbBr 2 single crystals show a red luminescence under uv excitation at temperatures below 200°K. Furthermore, PbCl 2 shows a yellow emission at temperatures below 40°K. The centers responsible for these emissions have been investigated by EPR measurements. These measurements indicate that due to uv irradiation Pb' Pb, (Pb' Pb) 2 and lead colloids are created. In PbCl 2, a hole center, possibly Cl ·cl, is created at temperatures below 50°K. Both the concentration of the Pb' Pb and the (Pb' Pb) 2 centers in PbCl 2 depends on the irradiation intensity; the concentration of the pair-centers increases with time after the irradiation is stopped. A similar dependence has been observed on PbBr 2 crystals cleaved and mounted in the dark, but in this case the influence of the irradiation appears to be smaller. The defects responsible for the red and yellow luminescence could be identified on the basis of temperature dependence measurements. The red luminescence of both halides is ascribed to excitation and decay of Pb' Pb or (Pb' Pb) 2 centers, and the yellow luminescence of PbCl 2 is associated with Cl ·Cl centers.