OPTICAL-TRANSITIONS OF THE H-CENTERS IN ALKALI-HALIDES

Comparison is made of the experimentally measured and theoretically ca lculated absorption spectra of the two types of intrinsic X(2)(-) hole centers (i.e., V-K and H centers) in several alkali halides (X denote s a halogen atom). The optical absorption spectra of these centers in NaCl, KCl, KBr, KI, and RbI are obtained employing the dichroic spectr oscopy. The geometric structure of the V-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 me thod, and the defect optical transition energies are calculated using the configuration interaction technique. It is shown that the latter i s essential for the correct representation of the wave functions of th e excited defect states and for the understanding of the nature of the electronic transitions associated with the defects. The analysis of t he spin density in the excited states of the H centers demonstrates th e delocalization of these states over several anions surrounding the X (2)(-) molecular ion. The existence of the optical transitions of the H center higher in energy than the UV band ascribed to the sigma(g)-si gma(u) transition is shown theoretically and confirmed experimentally. The difference in the optical properties of the H and V-K centers is discussed in terms of the geometric and electronic structure of the tw o centers.