Mechanism and kinetics of color center formation on epitaxial thin films of MgO

Electron stimulated desorption of oxygen atoms from the surface of epitaxially grown MgO films on Ag(1 0 0) produces characteristic losses of color centers in electron energy loss spectroscopy. The formation of five distinct loss features at 1.0, 1.3, 2.4, 2.8 and 3.4 eV have been observed that are induced by electron bombardment of the MgO surface. The possibility to erase these losses completely by simple exposure to O2 gas at room temperature suggests that they are predominantly located at or near the surface. From the dependence of loss intensities on electron exposure we conclude that the defects are generated directly by the electron impingement. The observed losses are tentatively assigned to electronic transitions associated with surface F centers (Fs) located at differently coordinated sites (surface, steps) and to M-centers. The characteristic steps of the cross-section at electron energies of 55 and 86 eV, which are close to binding energies of the Mg 2p and 2s energy levels in MgO, suggest a desorption mechanism via a multi-electron Auger decay process.