Configuration Coordinate Model for the F Center in KBr

Experimental absorption curves for the F center in KBr, after correction for K-band absorption, have been analyzed in terms of the semiclassical configuration coordinate model with linear and quadratic coupling. This model fits the absorption data well at both the low-temperature and high-temperature limits. The best fit to the data is found by varying five parameters: These are the force constants in the ground and excited states, the photon energy corresponding to the maximum in the absorption band (Em), the relative displacement of the minima of the adiabatic potentials for the ground and excited states, and the effective frequency of the vibration which is coupled to the ground electronic state. Consistent values of the parameters are found at both temperature limits. An expression is derived for the shift of Em with temperature. It is pointed out that not only is Em not identical with the average photon energy but that its temperature dependence actually has the opposite sign. The theory is tested by predicting the temperature shift of Em; after suitable corrections for thermal expansion the agreement with experimental data is surprisingly satisfactory.