Thermal Motion of Holes in Potassium Iodide

Previous studies of the anisotropic absorption of oriented VK centers (self-trapped holes) in KI showed that thermal reorientation occurs by a jumping motion in which the VK axis changes direction by 60^∘{} and is characterized by a thermal activation energy of 0.27 eV. An investigation of a possible motion along the VK center's axis, which cannot be detected by anisotropic absorption methods, is reported here. The thermal activation energy for the long-range diffusion of VK centers was measured in two ways: (a) by monitoring the isothermal decrease of the principal VK absorption band, and (b) by monitoring the luminescence which is the result of diffusing VK centers annihilating trapped electrons. The activation energy needed for diffusion was found to be the same, within experimental error, as that needed for reorientation. On this basis it is concluded that the dominant mode of motion for holes in the temperature range of these experiments is by a series of reorientation jumps.