FBand in Isotopically Enriched LiF

The effect of isotopic composition on the optical obsorption spectrum of the $F$ center in LiF has been investigated. The results of measurements on single crystals of x-ray irradiated ${\mathrm{Li}}^6$F (almost 96% lithium-6) and ${\mathrm{Li}}^7$F (almost 100% lithium-7) indicate that the average $F$-band width at half-maximum in the former is larger by about 3% at liquid-helium temperature, and by about 1% at room temperature. Theory shows that the half-widths in the two isotopic forms vary inversely as the fourth root of the mass of the alkali ions at low temperature, and as the temperature is increased the predicted half-width difference monotonically decreases to zero. The predicted half-width difference at low temperatures is about 4%, and at room temperature about 1%, both in reasonable agreement with experiment. At both room and liquid-helium temperatures the position of the $F$-band maximum in ${\mathrm{Li}}^7$F is slightly displaced to higher energy than in ${\mathrm{Li}}^6$F. The peak-position shift is in approximate agreement with the contributions arising from the difference in alkali mass and lattice constant of the two crystals as estimated at low temperature.