Effect of Optical and Thermal Bleaching on the uv Bands in Highly Pure and Mn-Doped KCl Crystals

In highly pure KCl crystals, × irradiated at room temperature (RT), the usual uv absorption bands at 212 and 230 nm are observed. The peak position of the 212-nm band when bleached with uv light shifts gradually to 195 nm, without any appreciable change in the intensity of this band. On thermal bleaching (in the range RT to 150°C) in the dark, the position and the intensity of the 212-nm band remain unchanged. When bleached thermally or optically with uv light, the peak position of the 230-nm band remains unaltered, whereas its intensity decreases. In uncolored KCl: Mn crystals, Mn introduces two uv absorption bands at 201 and 273 nm, having half-widths 0.36 and 0.15 eV, respectively. On × irradiation, the positions, intensities, and half-widths of these absorption bands remain unaltered, and a new absorption band at 223 nm appears. The intensity and half-width of the 223-nm band increases up to 25 h of × irradiation. Although thermally unstable, the centers responsible for the 223-nm band are fairly resistant to uv-light bleaching. An activation energy $E_a=0.54\mathrm{}\pm 0.02$ eV is obtained for the thermal bleaching of the centers that absorb in the 223-nm band. Qualitatively, the 223-nm band is similar to the $D$ bands observed by Ishii and Rolfe.