Interaction of Exciton with Electron Trapping Centers in Alkali Halide Crystals. 2. Quantum Yield for the U→F Conversion and Its Dependence on U-Center Concentration

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The quantum yield for the photochemical conversion of a U center to an F center as a function of illuminating photon energy has been investigated in KCl, KBr and their mixed crystals. For photons in the U-absorption band the yield increases with the illuminating photon energy. It has been determined that the conversion occurs with a maximum yield of nearly unity for photons both in the shorter wave-length tail of the U-absorption band and in the fundamental absorption tail-exciton band. The dependence of the yield of exciton-induced U→F conversion upon the temperature and U center concentration has also been studied. The yield of unity is independent of the concentration in the range higher than 1.0×10¹⁷/c.c.; however, it drops appreciably below 5×10¹⁶/c.c., indicating that the exciton is movable and its maximum length of migration is of the order of 3×10⁻⁶ cm.