Density-functional model cluster studies of EPR g tensors of F _s ⁺ centers on the surface of MgO

We report g tensors of surface color centers, so-called Fs+ centers, of MgO calculated with two density-functional approaches using accurately embedded cluster models. In line with recent UHV measurements on single-crystalline MgO film, we determined only small g -tensor anisotropies and negative shifts Δg≡g- ge for all Fs+ sites considered, namely, (001)-terrace, step, edge, and corner sites. The g values are very sensitive to the local structure of the defect: relaxation reverses the sign of Δg. However, accounting for the spin-orbit interaction either self-consistently or perturbatively yields very similar results. In addition to the values of the tensor components, their direction with respect to the surface was determined. In contrast to edges, significant deviations from ideal C2v symmetry were found for Fs+ centers at steps. Recent data on single-crystalline thin films are reevaluated in the light of these results.