Optical absorption of small polarons bound in octahedral symmetry: V− type centers in alkaline earth oxides

The optical absorption of V^–-type centers in alkaline-earth oxides is explained as a light induced transfer of holes between equivalent O^2- sites near 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 are consistent with the value of the ground state resonance interaction, known for MgO:V^–. The model furthermore predicts the existence of a relaxed stable excited state lying close to that determined experimentally for MgO:V^–. Transitions between nonequivalent orbitals at the equivalent O^2- sites lead to bands at higher energies than the previous ones, observed for all V^–-type centers. Generalisations of the treatment of these simple systems to optical absorption of free small polarons are given.