Oxygen-induced near-surface structural rearrangements on Ni{001} studied by shadow-cone-enhanced secondary-ion mass spectrometry

The surface structure of Ni{001} and the adsorption systems p(2×2) O/Ni{001} and c(2×2) O/Ni{001} have been studied by shadow-cone-enhanced secondary-ion mass spectrometry. The secondary Ni+-ion intensity has been measured as a function of the incidence angle of the primary Ar+-ion beam. The enhanced intensity features in the spectra are compared with results from a two-body-interaction calculation which uses the Moliére approximation to the Thomas-Fermi potential. For the clean Ni{001} surface the analysis indicates that the spacing between the first and second layer decreases from the bulk value of 1.76 to 1.68±0.06 Å. Moreover, the spacing between the second layers and the third layer remains nearly bulklike at 1.74±0.16 Å. For the oxygenated surfaces the O-Ni bond length is determined to be 1.96±0.05 Å, which corresponds to a height for the oxygen of 0.85 Å above the Ni substrate. The presence of oxygen is found to cause buckling in the second Ni layer in accord with previous low-energy-electron-diffraction observations. The magnitude of the buckling is found to be 0.26±0.12 Å and 0.20±0.10 Å for the p(2×2) and c(2×2) surfaces, respectively. These results suggest that the O-Ni bonding is highly localized and only modestly dependent upon coverage.