Formation of surface color centers at differently coordinated sites: MgO/Ag(1,1,19)

Abstract

In order to find criteria for discrimination between surface color centers at terraces and steps, and bulk color centers, the characteristic losses of these centers have been investigated by electron energy loss spectroscopy on an epitaxial MgO film grown on a Ag (1,1,19) surface. This film contains a significantly higher step density compared with a film grown on Ag(100). The generation of four distinct losses at 2.0, 2.4, 2.8, and 3.4 eV and a broad loss centered at 5.5 eV have been observed that are induced by electron bombardment of the MgO surface. The latter loss is attributed to bulk color centers ${(F}_{b}$ centers). By comparing the measured loss energies with experiments performed on MgO/Ag(100) and with theoretical literature data, the observed losses at 2.0 and 2.8 eV can be consistently assigned to transitions of color centers located on step sites, whereas those at 2.4 and 3.4 eV are attributed to terrace sites. The kinetics of color center formation during electron bombardment, as well as the annihilation of F centers by simple exposure to ${\mathrm{O}}_{2}$ gas at room temperature, was determined and compared for differently coordinated color centers. While all surface color centers follow the same kinetics, we show that the ${F}_{b}$ centers can only decay as long as the existence of surface color centers provides active sites for dissociating oxygen molecules. This corroborates the assignment of ${F}_{b}$ centers.