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(2X2) O/N i{001} and c(2X2) O/Ni{001} have been studied by shadow-cone-enhanced secondary-ion mass spectrometry. The secondary Ni+-ion intensity has b een measured as a function of the incidence angle of the primary Ar+-i on beam. The enhanced intensity features in the spectra are compared w ith results from a two-body-interaction calculation which uses the Mol iere approximation to the Thomas-Fermi potential. For the clean Ni{001 } surface the analysis indicates that the spacing between the first an d second layer decreases from the bulk value of 1.76 to 1.68+/-0.06 an gstrom. Moreover, the spacing between the second layers and the third layer remains nearly bulklike at 1.74+/-0.16 angstrom. For the oxygena ted surfaces the O-Ni bond length is determined to be 1.96+/-0.05 angs trom, which corresponds to a height for the oxygen of 0.85 angstrom ab ove the Ni substrate. The presence of oxygen is found to cause bucklin g in the second Ni layer in accord with previous low-energy-electron-d iffraction observations. The magnitude of the buckling is found to be 0.26+/-0.12 angstrom and 0.20+/-0.10 angstrom for the p(2X2) and c(2X2 ) surfaces,respectively. These results suggest that the O-Ni bonding i s highly localized and only modestly dependent upon coverage.