The optical anisotropy of cubic nickel‐iodine boracite due to quadrupole transitions

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Abstract
Measurements of the optical absorption of NiI boracite crystals for [100], [110], and [111] directions of light propagation reveal an unexpected angular dependence of the value of absorption coefficient and for the [110] direction also a dependence on the orientation of the plane of polarization of the incident light. These results as well as the existence of pronounced birefringence for the [110] direction of light propagation can be explained by the presence of quadrupole transitions. The results obtained are compared with Hellwege's theory of quadrupole transitions [1]. The functions which describe the angular dependence of the radiation probability for a cubic crystal with quadrupole transitions are found to be in agreement with the experimental results, if the theoretically determined functions [1] are taken in linear combination so as to fulfill the requirement of cubic symmetry.