Quasicontinuum Model of $F$-Aggregate Centers: Application to the $M^{\prime }$ Band

A quasicontinuum model of $F$-aggregate centers is introduced and used to calculate the low-lying states of the $F_2$ and ${F_2}^{\prime }$ centers in typical alkali-halide crystals. We have used our theoretical results to discuss the possibility that the $M^{\prime }$ absorption band is due to the ${F_2}^{\prime }$ center.