OPTICAL ABSORPTION OF V--TYPE CENTERS IN MgO AND CaO : SMALL POLARONS BOUND IN OCTAHEDRAL SYMMETRY

Abstract

The optical absorption of V--type centers (F+ antimorphs and related defects) in the alkaline-earth oxides is explained as a light induced transfer of holes between equivalent O2- sites next to cation vacancy type defects, the excitation acting against the self-induced trapping potentials. The interplay between symmetry-breaking hole-phonon- and symmetry-restoring resonance-interaction is formulated as a Pseudo-Jahn-Teller effect. The model explains the width, oscillator strength, position and resonance splitting of the bands. The parameters explaining these features also reproduce the position of a relaxed stable excited state and the resonance interaction between the vibrational ground States, known for MgO : V-. Transitions between nonequivalent orbitals at the equivalent O2- sites lead to bands at higher energies, observed for all V-.-type centers. Generalisations of the treatment of these simple systems to optical absorption of free small polarons are given. The detailed version of the paper is published in Zeitschrift fur Physik B [1].