PbClz and PbBr, single crystals show a red luminescence under uv excitation at temperatures below 200°K. Furthermore, PbCl* 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 .. . n-radlatlon Pb;,, (PbL& and lead colloids are created. In PbC12, a hole center, possibly Cl& is created at temperatures below 50°K. Both the concentration of the Pb;, and the (Pb& centers in PbClz 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 PbBrz 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;, or (Pb& centers, and the yellow luminescence of PbCI* is associated with Cl,& centers.

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SynopsisExperiments on an electric dipole centre, exhibiting multiple relaxation, in the natural mineral sodalite, are described. The concentration of the dipole centres is reduced upon X irradiation, whereas simultaneously colour centres and paramagnetic centres arise. Thermal bleaching restores the original concentrations. The interrelationships between these centres are established.A model is proposed in which the dipole centre is ascribed to an interstitial monovalent metal ion (say a Na + ion) acting as a charge compensator for an A13÷ ion, substituted for a Si 4+ ion. This A1-Na complex may be destroyed by X rays, yielding an electron trapped at the sodium ion and a hole trapped at a nonbridging oxygen ion, adjacent to the A13÷ ion, to which both the optical and paramagnetic properties of the X ray induced centres are attributed.

Microwave absorption in smoky quartz mono-crystal is ascribed to resonance transitions of trapped electrons between initially eonfigurational degenerated states, which are Stark-splitted by a polarizing electric field.

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