Configuration Coordinate Curves forF-Centers in Alkali Halide Crystals

Abstract

Measurements have been made of the $F$-center absorption band in KCl, KBr, KI, NaCl, and LiF in the range from 4°K to 300°K. Analysis of these data shows that the important ionic vibrational frequency influencing the $F$-center is considerably lower than that of the lattice, indicating that a configuration coordinate curve model may be the appropriate one to use, rather than the continuous dielectric model. Using the peak of the $F$-center emission bands as determined by Botden, van Doorn, and Haven, and the present absorption data, configuration coordinate curves are computed for the ground and excited state curves of $F$-centers in KCl, KBr, KI, and NaCl. Using these curves, predictions of the emission band-widths are found to be in agreement with experiment except for the case of KCl. The low efficiency of $F$-center luminescence may be understood by applying the criterion for luminescence proposed by Dexter, Klick, and Russell to these configuration coordinate curves.