Abstract
Several possible ionization mechanisms of excited centers () in alkali halides are discussed. A lattice tunneling model for the low-temperature ionization is proposed which implies considerable rearrangement of the lattice as a condition for a radiationless electron transfer from the center to a virtual polaron center in the crystal. This model rests on a configurational diagram based on experimental absorption and emission data. A recent reaction-rate theory of electron hopping in polar crystals is applied to calculate the rate constant of the ionization in the temperature range 10-160 K by the use of reasonable values of four parameters: the LO vibration frequency , the reorganization energy of the lattice, the reaction heat at zero temperature, and the resonance energy of the electron transfer. In this way good agreement between the theoretical results and the available experimental data for is obtained. In addition, an independent estimation of the "average" resonance energy, based on a simple donor-acceptor model for the electron transfer, is found to agree very well with the values of fitted to the experiment. The average values of the electron tunneling distance and the average donor-acceptor (-center—polaron-center) separation derived from this model seem also to be quite reasonable. Some implications of the theory concerning the reverse process of electron transfer from a free polaron to an empty ion vacancy are also discussed.
- Received 1 December 1981
DOI:https://doi.org/10.1103/PhysRevB.26.6936
©1982 American Physical Society