Assignment of Photoluminescence Spectra of MgO Powders: TD-DFT Cluster Calculations Combined to Experiments. Part II. Hydroxylation Effects

This paper deals with the assignment of photoluminescence excitation spectra of hydroxylated MgO powders thanks to a combined theoretical and experimental approach. Experimentally, despite the numerous types of hydroxyl groups formed upon hydroxylation of bare MgO surfaces, only three types of luminescent species are observed. TD-DFT excitation energy calculations are performed on hydroxylated MgO clusters modeling planes, edges, corners, kinks, and divacancies. The results indicate that the photoluminescence excitation properties do not depend primarily on the coordination of oxygen Of OLCH groups (LC= low coordination), which can be satisfactorily explained by theoretical calculations. The hydroxylation state of the surface can thus be more accurately followed by the thermal evolution of the bands characteristic of bare surface irregularities. In addition, OLCH groups are themselves directly implied in the excitation process only if they are located on convex areas of the surface and if L <= 3, L being defined as the number of magnesium neighbors. Otherwise, the observed excitation modes of OH groups rather correspond to perturbations of excitation modes of the related dehydroxylated surface. The emission process is followed experimentally by lifetime measurements, which enable one to visualize energy transfer between surface species.