Color Centers in Alkali Halides Containing NO2— Ions in Small Concentration

Alkali halide single crystals (KBr, KCl, NaBr, and NaCl) containing NO2— ions at various concentrations, from 0.5 to 0.007 mole percent, were grown by the Kyropoulos method. At high NO2— concentrations the typical NO2— band appears in the absorption spectrum with peak wavelengths between 280 and 260 mμ depending on the nature of the base material. The production of F-bands by exposure to x-rays is completely suppressed in these crystals at room temperature and greatly inhibited at —190°C. Instead of the F-band, a continuous band (C-band) is produced with a wavelength nearly coincident with the NO2— band, but not showing at low temperature the vibrational structure characteristic of the latter. Other less important new absorption bands appear in the red and the ultraviolet. At lower NO2— concentrations F-bands begin to appear in competition with the C-bands. The bleaching of the various absorption bands by light absorbed in them is investigated and discussed. NaCl seems to differ from the other halides in so far as in this crystal the production of the F-band is greatly enhanced by NO2— in concentrations which in KBr greatly inhibit the formation of F-centers.