Optical transitions of the H centers in alkali halides.

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Comparison is made of the experimentally measured and theoretically calculated absorption spectra of the two types of intrinsic ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ hole centers (i.e., ${\mathit{V}}_{\mathit{K}}$ and H centers) in several alkali halides (X denotes a halogen atom). The optical absorption spectra of these centers in NaCl, KCl, KBr, KI, and RbI are obtained employing the dichroic spectroscopy. The geometric structure of the ${\mathit{V}}_{\mathit{K}}$ and H centers in NaCl, KCl, and KI are determined by optimizing the total energy of the molecular cluster embedded in the crystalline lattice using the Hartree-Fock method, and the defect optical transition energies are calculated using the configuration interaction technique. It is shown that the latter is essential for the correct representation of the wave functions of the excited defect states and for the understanding of the nature of the electronic transitions associated with the defects. The analysis of the spin density in the excited states of the H centers demonstrates the delocalization of these states over several anions surrounding the ${\mathit{X}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion. The existence of the optical transitions of the H center higher in energy than the UV band ascribed to the ${\mathrm{\ensuremath{\sigma}}}_{\mathit{g}\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{\sigma}}}_{\mathit{u}}$ transition is shown theoretically and confirmed experimentally. The difference in the optical properties of the H and ${\mathit{V}}_{\mathit{K}}$ centers is discussed in terms of the geometric and electronic structure of the two centers.