Interaction of Exciton with Electron Trapping Centers in Alkali Halide Crystals. 3. Exciton Induced Photoconductivity of U center

Photoconductivity of KCl and KBr crystals containing U centers has been investigated as a function of illuminating photon energy, in both the regions of U absorption and the fundamental absorption bands of host crystals. The maximum conductivity occurs where the fundamental absorption starts. The photoconductivity curve is similar to the spectral distribution of the quantum efficiency of U→F conversion as reported previously. The wavelength of light which enhances the maximum conductivity shifts to the longer wavelength side if the U center concentration is below 5×10 16 /cc. This fact is in agreement with that of the quantum yield dependence on the concentration. The behaviour of Br-ion band in KCl crystal upon the photoconductivity was also investigated and it was concluded that localized excitons which may be considered to be produced by photons absorbed in Br band, have the same property as the free excitons in the host crystal so far as the effect on U centers is concerned.