Photoequilibrium between KClFandF′Centers at 80°K

Abstract

The illumination of $F$ centers in KCl at 80°K establishes a photoequilibrium between $F^{\prime }$ and $F$ centers. The equilibrium transformations are reversible and are of the same magnitude for additively colored crystals and crystals x-rayed at 80°K and 300°K. The photostationary ratio of $F^{\prime }$ to $F$ centers varies with the logarithm of the incident light intensity. Transitions between equilibrium levels approximate first-order processes, controlled by the rate of light absorption in the $F$ and $F^{\prime }$ bands. A kinetic model is proposed in which the elementary steps are the photoionization of $F$ and $F^{\prime }$ centers, and electron capture at $F$ centers and negative-ion vacancies. The variation of light intensity and spectrum through the crystal is accounted for by assuming that temporal equilibrium is maintained in local regions and integrating the results over the crystal volume. The photoionization quantum efficiency of the $F$ center at 80°K is calculated to be 0.020±0.003; this quantity is one-half of the photobleaching quantum efficiency. A value of 0.016±0.005 is obtained for the ratio of the electron-capture cross section of the $F$ center to the chloride-ion vacancy.