Color Centers in Cesium Halide Single Crystals

Color centers have been investigated in the CsCl-type alkali halides. Cesium chloride, bromide, and iodide single crystals were grown from the melt and CsCl crystals also from solution. Coloration was produced by 130-kv x rays, 3.0-Mev electrons, and by electrolysis. In CsI coloration resulted from electrolysis only. The absorption of uncolored and colored crystals has been measured from 0.175 to 3.5 {μ} at 25^∘{}, -78^∘{}, and -190^∘{}C. After coloration all three crystals show one strong band in the visible (near infrared for CsI) and several weaker bands at shorter and longer wavelengths, which shift with temperature change. The spectral positions in m{μ} for a number of the bands at -190^∘{}C are: The strongest band (λ₆)behaves similarly to the F band in the NaCl-type alkali halides. The half-width of the band (0.20 to 0.23 ev at -190^∘{}C) and its increase with temperature, the shifting of the band maximum toward longer wavelengths upon warming to room temperature (by {~}0.08 ev), and the conversion by bleaching with light into other bands support the assignment of this band as the F band. The spectral position of the band maximum approximately follows the Mollwo relation (λₘₐₓ=constdⁿ, where d=interionicdistance and $n=2.5$).Bleaching experiments suggest the assignment of the band λ₁₀ as the M band and the bands between the F and M bands as R bands. The origin of the ultraviolet bands is still uncertain.