Studies of Color Centers in Polycrystalline Compressed Pellets of Alkali Halides

Abstract

Compressed and uncompressed powders of several alkali halides (KCl, KBr, KI, CsI) have been colored by x-rays and by chemical addition and their spectra measured. The x-ray darkenability of pellets is greater than of single crystals, with respect to F, V2, and V3 bands. The F centers are relatively unstable optically; their bleaching is rapid and complete and does not give rise to appreciable K, M, R and N bands, in contrast to F centers in single crystals. After F irradiation the pellets still exhibit intense, well-resolved V2 and V3 bands and, upon dissolving in water, liberate free halogen. The relative intensity of the F and V bands may be controlled by varying the x-ray or F-light exposure. KI pellets may be additively colored at room temperature with iodine. In general V bands may be produced by adsorbing halogen onto the crystalline surface and are more stable in the compressed pellets than the unconfined powder. By virtue of their different physical makeup from a single crystal the pellets exhibit the above color-center phenomena to a larger degree than single crystals.