Studies of the Optical Absorption Spectrum of an Interstitial Co⁺⁺ Ion in KCl Crystals

Abstract

The optical absorption spectrum of the interstitial Co⁺⁺ ion in KCl observed by Nasu and Kuwabara is studied theoretically in the framework of the crystalline field theory. The crystalline field parameters D_q and B are determined (D_q=325 cm⁻¹, B=734 cm⁻¹). The absolute value of the oscillator strength of the visible absorption peak is calculated, by using Van Vleck molecular orbital model. The calculated f value by using the 3d and 4p orbitals of the Co⁺⁺ ion and the σ orbitals of the ligands is in good agreement with the experiment. In this case, the Co⁺⁺ 4p orbitals which are admixed with the 3d orbitals play an important role in the intensity. The mixing ratio of the 4p orbitals determined from the observed f value is about 0.35. If K⁺ vacancies are present near the Co⁺⁺ ion, this value can be obtained by the perturbation due to the crystalline field of lower symmetry, but if the symmetry of the crystalline field is tetrahedral, such a large value cannot be obtained.