First-principles study of the optical transitions of F centers in the bulk and on the (0001) surface of α-Al2O3 -: art. no. 054109

The electronic structure and spectroscopic features of the optical spectra of oxygen vacancies in the bulk and on the (0001) surface of alpha-Al2O3 have been studied by first-principles methods. The effect of oxygen vacancies on the crystalline structure has been determined by the appropriate atomic structure optimization carried out using a periodic model and density functional theory (DFT) calculations. Both, neutral (F center) and charged (F+ center) oxygen vacancies have been considered. Optical transitions arising from the excitations of the electron(s) trapped in the F+ and F centers, both on the surface and in the bulk of the material, have been studied using a suitable embedded cluster model approach. The cluster geometry, including the relaxation around the vacancy, has been taken from the periodic calculations. The transition energies and intensities have been obtained by applying the explicitly correlated multiconfigurational second-order perturbation theory method and also time-dependent DFT approaches. The present results are in good agreement with available experimental data for the bulk and provide a guide to interpret forthcoming experiments involving the spectroscopy of oxygen vacancies present in irradiated or defective alpha-Al2O3(0001) surfaces.