Lifetime of the ExcitedFCenter

An investigation was carried out on the excited state of the $F$ center in various alkali halide crystals as a function of temperature. Results are presented on the time dependence of photoconductivity and luminescence following excitation by a very short, ${10}^{-8\mathrm{}}$ sec, pulse of light. The data can be interpreted in terms of a simple energy-level diagram of the $F$ center consisting of a ground state and an excited state. Estimates are given of the depth of the excited state below the conduction band $\Delta E$, the frequency factor for thermal ionization $\frac{1}{{\tau }_0}$, and the radiative lifetime ${\tau }_R$, for KCl, KBr, KI, and NaCl. The radiative lifetime ${\tau }_R$ depends upon the alkali halide, but may be as long as ${10}^{-6\mathrm{}}$ sec. Suggestions are made for understanding this long lifetime in view of the large oscillator strength of the $F$ center. The effect of $F$-center concentration and also of an admixture of $M$ centers is discussed.