Electronic States of theFCenter in Alkali Halide Crystals

The possible importance of $F$-center states in alkali halides associated with the energy-band minimum at $X_3$ in the Brillouin zone is discussed. The coexistence of these states with the "traditional" $F$-center states associated with ${\Gamma }_1$ may lead to new and interesting phenomena. In particular, it is speculated that the discrepancy between radiative lifetime and ground-state repopulation time observed by Fulton in the CsF $F$ center can be resolved by including $X_3$ $F$-center states.