Ab initio study of the optical transitions on low-coordinated sites of an intermediate F center: The Fs+(OH)− center on MgO(100) surface

The optical transition energies of a point defect in MgO(100), consisting of an oxygen vacancy with one trapped electron and an OH− group adsorbed nearby the surface oxygen vacancy (the Fs+(OH−) center), have been studied by means of ab initio multiconfiguration second order perturbation theory (CASPT2) and Density Functional (DFT) calculations on embedded cluster models. We find that a method that takes explicitly into account electronic correlation effects is required to properly predict the first transition energies. The lowest transition energy of the Fs+(OH−) center on a terrace site is very similar to the one reported in previous studies of F+ centers on a regular MgO(100) surface (Fs+ center). Experiments have difficulty distinguishing between these two kinds of point defects based solely on the optical spectra. We have also considered the situation where the vacancy is located at low-coordinated sites like steps and corners. The results show that the loss of coordination decreases the transition energies in agreement with what has been reported in previous studies for “classical” Fs+ centers.