Influence of the surface structure on the D2O and H2 18O adsorption on MgO(1 0 0) thin films studied by electron stimulated desorption

The reactivity of D2 16O and H2 18O with thin films of MgO(1 0 0) at room temperature was studied by electron stimulated desorption time of flight (ESDTOF) mass spectrometry. The MgO(1 0 0) thin films were grown on Si(1 0 0) at 1000 K by RF sputtering in an Ar (90%) and O (10%) atmosphere. The energy of the incident electrons ranged from 5 to 600 eV and the incident electron current on the sample was 100 nA. When the surface is exposed to D2O, the peaks of the ESDTOF surface ion spectra correspond to H+, D+, O+ and OD+ desorbed ions. After H2 18O exposure, H+, 16O+, and 18O+ desorbed ions are observed. Both 16O+ and 18O+ ions have a bimodal structure, which corresponds to oxygen ions with most probable kinetic energies of 5.3±0.5 and 9.0±0.9 eV. This bimodal structure is strongly affected by the energy of the incident electrons as well as by the adsorbed species. At electron energies lower than 250 eV the state with the lowest kinetic energy almost disappears. The two peaks at 5.3±0.5 and 9.0±0.9 eV of the bimodal structure are consistent with those observed in previous studies on bulk MgO(1 0 0) at 6.0±0.6 and 9.0±0.9 eV energies. The ion yields of desorbed ions for thin films and bulk samples are dramatically different. These results are discussed with respect to the surface reactive adsorption sites as well as to the secondary electron yield.