ANIONIC B CENTERS OF SILVER IN ALKALI HALIDES

It is proposed that negatively charged silver ions on anion sites are responsible for the B bands found in silver doped alkalihalides. Experimental investigations confirming this model of the B center are presented. Optical absorption and emission of B centers were measured in seven alkalihalides in the temperature range from 450 to 4 °K. Configuration coordinate diagrams were obtained for KCl, KBr, and KI. A chemical method was used to determine the charge of the B center. Thermal or optical excitation causes the B centers to dissociate into neutral silver atoms and F centers. The Ag0 centers are bound to interstitial positions at low temperature. At high temperature they collect together forming colloidal centers. Tetragonal BA centers were formed during optical bleaching of the B band in mixed crystals of the type KCl+ΔNaCl. Their absorption and emission spectra, optical orientation, and polarized emission were investigated. These properties are shown to be similar to those of FA centers. BA fluorescence polarized almost completely in the [100] direction was observed. A Jahn-Teller splitting of the B absorption band was resolved in RbCl at low temperature. The splitting confirms the configuration 5s2 for the Ag- ion. The results are compared with those for the isoelectronic centers In+ and Sn++. Thin films of alkalihalides containing small amounts of silver, copper, or thallium were condensed simultaneously with alkali vapor. New bands in the UV region were found, possibly due to Cu- and Tl- centers. © 1968 Springer-Verlag.