Optical transitions in ZnS type crystals containing cobalt

The optical properties of ZnS crystals, ZnS and (Zn,Cd)S phosphors activated by cobalt were studied in the temperature range from 4.2°K to 300°K. The absorption bands with an energy of 1.75 and 0.85 eV split when the crystals are cooled. However, the absorption at 0.4 eV is more pronounced at 76°K than at the temperature of liquid helium. The infrared emission due to cobalt at 0.4 eV increases by cooling the crystal and shows some structure at low temperature. This emission can be excited by irradiation into the absorption bands at 1.75 and 0.85 eV.Additional irradiation by short wavelength light (2.5 eV ≦ E ≦ 3.2 eV) at low temperatures changes the absorption spectra and the emission spectra : (1) The characteristic excitation peaks of Co⁺ are reduced, whereas an additional absorption with a maximum at about 1.3 eV occurs. (2) The infrared emission due to Co⁺⁺ at 0.4 eV decreases, but a new emission within an energy range of 0.6 eV ≦ E ≦ 0.9 eV appears, generated in Cu⁺⁺ centres.The changes of optical properties by additional radiation and by cooling the crystals are explained by charge transfer processes Co⁺⁺ ⇄ Co⁺ and Cu⁺ ⇄ Cu⁺⁺.