Abstract

This is an extension of our earlier work on the F center¹. We had calculated the thermal ionization rates of excited F centers (F^*) in KCl using experimental data on the F center photoionization efficiency and luminescent lifetime. When plotting them versus the respective reciprocal temperatures, considerable deviations from the Arrhenius temperature behavior were found to occur below 100 K leading to a nonvanishing residual ionizability at low temperature. These were suspected to result from a strong lattice tunneling at the lower temperatures. A reaction-rate theory was applied for a quantitative analysis which expresses the rate constant in terms of four adjustable parameters: vibrational frequency ω, lattice-reorganizational energy E_r, reaction heat Q at 0 K, and electron-exchange integral V₁₂.² The ionization transition was regarded as the one in which the F^* electron transferred to a polaron state by virtue of the strong interaction with a promoting mode. Very reasonable values were obtained for the fitting parameters, as the theory was compared numerically with the experiment. V₁₂ was also estimated independently within the BWK approximation resulting in a remarkable agreement between fitted and computed values.

© 1984 Optical Society of America