Optical properties of surface and bulk F centers in MgO from ab initio cluster model calculations

The optical transitions of neutral and charged oxygen vacancies in the bulk and on the surface of MgO have been studied by means of first principles calculations. Using rather large cluster models representing the MgO F+ and F centers, oxygen vacancies with one and two electrons trapped in the cavity, respectively, we have performed extensive configuration interaction calculations on the ground and excited states. The computed optical transitions for bulk F and F+ centers occur at a similar energy, in agreement with the experimental measures. The absolute values of the transitions, approximate to 6 eV, are overestimated by about 1 eV compared to the experimental value of approximate to 5 eV, mainly because of basis set limitations. On the surface, the F-s and the F-s(+) centers are found to absorb photons of lower energy than in the bulk. The expected transitions on the surface are smaller by about a factor of 2 than in the bulk. Thus, transition energies in the range 2-2.5 eV are predicted by the calculations. The results are compared with the few existing measurements of optical transitions at the surface of MgO. (C) 1998 American Institute of Physics.