Isotope splitting of the F-center in-gap mode in KI and KBr

High-resolution measurements at the F-center in-gap mode in KI and KBr have revealed a threefold structure in the absorption spectrum with lines at 82.62 +/- 0.02, 81.98 +/- 0.02, 81.19 +/- 0.05 cm(-1) and at 99.60 +/- 0.03, 99.07 +/- 0.04, 98.50 +/- 0.05 cm(-1), respectively. This structure Is interpreted as due to the presence of two stable isotopes, K-39 and K-41, in the crystal. In a three-dimensional-model calculation which uses as parameters changes in the force constants between the defect and the first nearest neighbors (A01) and changes in the force constant between the first and fourth nearest neighbors (A14), we found that the in-gap mode frequencies are extremely sensitive to the value of the latter. The position and splitting in the absorption lines can be explained in the calculation by using A01 = -0.50, A14 = -0.060 for KI and A01 = -0.50, A14 = + 0.002 for KBr. Stress experiments on the strongest lines in KI are in agreement with our model. Extending the calculation to Cl- and Br- centers in KI, it is shown that the measured isotope splitting for Cl-35, Cl-37 and Br-79, Br-81 centers can be explained by taking into account not only changes in A01 but also changes in A14.