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
Dq and
B are determined (
Dq=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 3
d and 4
p orbitals of the Co
++ ion and the σ orbitals of the ligands is in good agreement with the experiment. In this case, the Co
++ 4
p orbitals which are admixed with the 3
d orbitals play an important role in the intensity. The mixing ratio of the 4
p 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.
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