Excited State of theFCenter in KI

Abstract

The excited state (here called $F^{*}$) of the $F$ center in KI was studied using a ruby laser as an excitation source. The absorption spectrum of the crystal was measured immediately before, during, and after an excitation pulse by using an auxiliary light source and monochromator. From these data the absorption spectra of the $F^{*}$ center and the $F^{\prime }$ center were obtained. The fraction of the total number of centers maintained as $F^{*}$ centers by the excitation pulse and the $F\to F^{\prime }$ conversion during the pulse are interpreted in terms of a kinetic model which contains an important additional term beyond those in the usual models. The new term is due to the overlap between the strong absorption of the $F^{*}$ and the ordinary fluorescence of the $F^{*}$. A nonradiative process ($2F^{*}\to F+\mathrm{electron}+\mathrm{vacancy}$), which can now take place in addition to the ordinary fluorescence, severely limits the population of $F^{*}$ which can be maintained. The kinetics have been studied as a function of $F$-center concentration and laser intensity.