FCenter in Ionic Crystals: Semicontinuum Polaron Models

Abstract

The states of the $F$ center are considered on the basis of models which treat the movement of the nearest neighbors to the $F$ center and the $F$ electron in a self-consistent manner. The lattice is described in terms of a classical ionic-crystal theory, and the $F$ electron is treated according to the semicontinuum polaron theory. The absorption energy, the emission energy, the lifetime of the first excited state, the thermal activation energies, and the Huang-Rhys factor are evaluated for two models (Hartree and quasiadiabatic) which differ in the evaluation of the optical polarization. It is shown that the Hartree semicontinuum polaron model agrees best with the experimental results for NaCl, KCl, Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$. In addition, these models show that among the above quantities the thermal activation energies and the lifetime of the first excited state are most sensitive to the physical content of a given model.