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

We demonstrate a dramatic dependence on the oxygen coordination of the calculated optical absorption and luminescence energies of low-coordinated sites at the surfaces and in nanoclusters of MgO. The calculations for $(\mathrm{MgO}{)}_{256}$ cubic nanoclusters were performed using an embedded molecular cluster model and both semiempirical and ab initio Hartree-Fock methods. The optical-absorption energies were calculated using the configuration interaction technique for single (CIS) and double electronic excitations. The luminescence energies were calculated using the CIS method. The low-coordinated sites included corners, kinks, step and cluster edges, and corner vacancy defects. We have also studied the zigzag steps and monatomic steps at infinite surfaces using a periodic density-functional theory method and a plane-wave basis 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 decreases. 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-coordinated surface sites could be used to study molecular adsorption at surface sites, photocatalytic surface processes and desorption induced by electronic transitions.