Electronic Structure of theMCenter in LiCl and LiF

Experimental work of the past few years has fairly conclusively established that the $M$ center in alkali halides consists of two $F$ centers bound together at neighboring halide sites. Using this model, the wave functions and energy levels of the $M$ center in LiCl and LiF are calculated in the Heitler-London approximation. The $F$-electron wave functions used in the calculations are of the vacancy-centered type. The finite extension of the neighboring alkali ions is treated but all other ions are considered to be point charges. The results of the calculations show that there is a transition energy which corresponds closely to the observed $M_1$ band. In addition, there are numerous possibilities for other transitions, all giving energies which cluster in the vicinity of the main $F$-band energy. The results for the $M_1$ band follow the empirical Ivey relationship quite well.