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 were investigated by electron energy loss spectroscopy of an epitaxial MgO film grown onto a Ag(1,1,19) surface. This film contained a significantly higher step density as compared with a film grown on Ag(100). The generation of 4 distinct losses, at 2.0, 2.4, 2.8 and 3.4eV, and a broad loss centered at 5.5eV were observed. These were induced by electron bombardment of the MgO surface. The latter loss was attributed to bulk color centers (Fb centers). By comparing the measured loss energies with experiments performed on MgO/Ag(100) and with published data, the observed losses at 2.0 and 2.8eV could be attributed to transitions of color centers located at step sites, whereas those at 2.4 and 3.4eV were 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 O at room temperature, were compared for differently coordinated color centers. It was found that, while all of the surface color centers exhibited the same kinetics, the Fb centers could decay only as long as the existence of surface color centers provided active sites for dissociating O molecules. This corroborated the assignment of Fb centers.

Formation of Surface Color Centers at Differently Coordinated Sites - MgO/Ag(1,1,19). J.Kramer, C.Tegenkamp, H.Pfnür: Physical Review B, 2003, 67[23], 235401 (7pp)