OXYGEN STRUCTURE ON NI(100) USING LOW-ENERGY LI+, NEGATIVE RECOIL AND H+ IONS

The c(2 x 2)O structure on Ni(001) has been probed by low energy Li+, negative recoil and H+ ion scattering. Li+ ion scattering has the ability to derive structural information directly from experimental data and from simple calculations on the basis of shadowing and blocking effects. Experimental results obtained by analyzing the incident angle dependence associated with shadowing of Ni by oxygen confirm that the O atoms are located in symmetrical fourfold hollow sites, though off-centre positions cannot totally be ruled out. The proper consideration of a relaxation of the top Ni layer proves to be essential for the accurate determination of the adsorption height of oxygen. The perpendicular height of oxygen was determined to be d perpendicular-to = 0.84 +/- 0.08 angstrom and the first interlayer spacing d12 is 1.86 +/- 0.05 angstrom, i.e. substrate layer relaxation is 0.10 angstrom (5.7% outward relaxation). The low energy recoil process is extremely sensitive to the electronegative atoms, and the detection of the recoiling species ensures that the adsorption site is probed directly. The angular distribution of negative ions used as a direct measure of the adsorption site. Hydrogen scattering experiments at low energy yield similar information to its medium energy (MEIS) and high energy (RBS) analogues by the appropriate choice of scattering geometries to probe the surface structure. Experimental results indicate that these two approaches are good complementary tools with LEIS for O adsorption studies. The value of the O height was independently obtained to be 0.75 +/- 0.10 and 0.8 +/- 0.2 angstrom respectively. The good agreement between the O adsorption heights obtained from the different methods lends confidence to the O height value and the reliability of the experimental and computational procedures.