Optical properties and transformation mechanism of oxygen centres and their aggregates in CaF2 crystals

Oxygen-vacancy dipoles and dimers in CaF2 crystals have been studied ab initio at DFT level and with the shell model using pair potentials. The calculated dipole reorientation barrier is 0.64 eV and the activation energy for diffusion of the dipoles is 1.61 eV. Optical absorption of O2--VA dipole have been calculated with TD DFT and identified with experimental absorption bands, which appeared to have complex structure. The photodissociation mechanism of the dipole is discussed. Several configurations of the dimer (O2--VA)2 were calculated. The association energy for the most favourable one is 0.48 eV.