Studies on Z 2 -Centers in Alkali Halide Crystals

Optical and thermal properties of Z 2 -centers and related photochemical processes have been studied for additively colored KCl, KBr and NaCl crystals doped with Ca, Sr and Ba ions. The peak wavelengths and half-widths of the Z 2 - and Z 3 -bands were determined. The Z 2 -absorption peak shifts toward the long wavelength side with increasing atomic mass of the divalent ions, increasing lattice spacing of the host crystal, and slightly with increasing temperature. The Z 2 -center in Ba-doped crystals always exhibits two absorption peaks. KCl doped with Sr was studied epecially extensively, as specimens showing only the Z 2 -band in the spectral range of 220 mµ to 1200 mµ were obtained; absorption and emission spectra were measured as a function of temperature and a configuration coordinate diagram of the Z 2 -center were drawn using experimental data. As a most plausible model compatible with experiments, an interstitial alkaline earth atom is proposed for the Z 2 -center and a positively ionized Z 2 -center for the Z 3 -center.