A comparison was made of the experimentally measured and theoretically calculated absorption spectra of the 2 types of intrinsic X2- hole center (VK and H) in these alkali halides. Here, X denoted a halogen atom. The optical absorption spectra of these centers were obtained by means of dichroic spectroscopy. The geometrical structures of the VK and H centers in NaCl, KCl and KI were determined, by optimizing the total energy of molecular clusters which were embedded in the crystalline lattice, using the Hartree-Fock method. The defect optical transition energies were calculated by using the configuration interaction technique. It was shown that the latter was essential for the correct representation of the wave functions of the excited defect states and for an understanding of the nature of the electronic transitions which were associated with the defects. An analysis of the spin density in the excited states of the H centers demonstrated a delocalization of these states over several anions surrounding the X2- molecular ion. The existence of optical transitions of the H center, which were higher in energy than the ultra-violet band, was predicted theoretically and confirmed experimentally. Differences in the optical properties of the H and VK centers were explained in terms of the geometrical and electronic structures of the centers.

A.L.Shluger, V.E.Puchin, T.Suzuki, K.Tanimura, N.Itoh: Physical Review B, 1995, 52[6], 4017-28