Spectroscopy of low-coordinated surface sites: Theoretical study of MgO

We demonstrate a dramatic dependence on the oxygen coordination of the calc ulated optical absorption and luminescence energies of low-coordinated site s at the surfaces and in nanoclusters of MgO. The calculations for (MgO)(25 6) cubic nanoclusters were performed using an embedded molecular cluster mo del and both semiempirical and ab initio Hartree-Fock methods. The optical- absorption energies were calculated using the configuration interaction tec hnique for single (CIS) and double electronic excitations. The luminescence energies were calculated using the CIS method. The low-coordinated sites i ncluded corners, kinks, step and cluster edges, and corner vacancy defects. We have also studied the zigzag steps and monatomic steps at infinite surf aces using a periodic density-functional theory method and a plane-wave bas is set. For both the nanoclusters and infinite surfaces, the results show a consistent significant reduction of the exciton excitation energies and of the luminescence energies of relaxed excitons as the oxygen coordination d ecreases. We also demonstrate the possibility of exciton transfer from the sites with higher coordination to those with lower coordination and finally to the localization centers. Selective optical excitation of low-coordinat ed surface sites could be used to study molecular adsorption at surface sit es, photocatalytic surface processes and desorption induced by electronic t ransitions. [S0163-1829(99)07203-3].