Magnetic Circular Dichroism of theFBand in KF and Quenching ofF-Center Spin Polarization by Optical Pumping

The magnetic circular dichroism (MCD) of the $F$ absorption band in KF has been detected. The field and temperature dependence of the MCD signal yielded $g_{\mathrm{orb}}\sim 1$ and $\Delta =-3.2\mathrm{}\pm 0.4$ meV for the orbital $g$ factor and spin-orbit splitting of the excited state of the $F$ center. The effect of unpolarized optical pumping on the MCD signal was investigated, with the result that at the maximum power level used (∼ ${10}^{15}$ photons/sec) approximately 75% of the signal was quenched. The data have have been interpreted using a simple model involving a small (efficiency $\epsilon \sim 0.01$) spin-memory loss while the $F$-center electron is in the excited state. The fact that the spin polarization of the $F$ center can be completely quenched if the pumping intensity is high enough is sufficient to explain the absence of a spin-dependent contribution to the magnetic circular polarization of the $F$-center emission observed by Fontana and Fitchen.