Stark Effects inF-Center Emission

The effect of an applied electric field on the emission spectrum of the $F$ center has been measured in the range 5-50 kV/cm and 2-150°K for six alkali halides (KCl, KF, RbCl, NaCl, NaF, and CsF). The changes induced by the field are quadratic in the field strength and isotropic with respect to the crystalline axes, and at $T\lesssim 30°\mathrm{K}$ are as follows: an enhancement of the emission polarized parallel to the field direction with a compensating decrease of the emission polarized perpendicular to the field, a red-shift of the band in both polarizations, and a broadening in certain cases. At higher temperatures the polarization and broadening diminish. The effects in CsF are anomalously small. In several crystals, field-induced quenching of the emission due to Schottky or field ionization also appears. These results are analyzed in terms of Stark mixing in the relaxed excited state of the $F$ center. The model assumes that the relaxed states consist of nearly degenerate $2p$- and $2s$-like states which are strongly mixed by crystal-field fluxtuations on the order of 200 kV/cm. For KCl, the resulting $2s^{\prime }$ mixed state is lower in energy by 0.017 eV and has approximately 40% admixture of the $2p$ states. This mixed-state model is able to give a semiquantitative explanation for the Stark effects observed, as well as a consistent explanation for the magnitude and temperature dependence of $F$-center radiative lifetimes and other excited-state phenomena.