Multilayer relaxations of Ni(110): New medium energy ion scattering results

Recent ion scattering and LEED measurements of the clean Ni(110) surface structure show a significant difference in the magnitude of the first layer relaxation. Ion scattering has measured the first layer contraction to be 4.5% while LEED I–V analyses have resulted in measurement of an 8.5% contraction. In an attempt to resolve this apparent discrepancy we have undertaken a careful reexamination of the clean Ni(110) surface with medium energy ion scattering using the technique of channeling and blocking. The experiment was conducted in a UHV chamber with a toroidal electrostatic analyzer using 110 keV protons. Our analysis was based on comparisons of measured surface blocking curves in two scattering geometries with full crystal Monte Carlo simulations. These simulations include variations of the interlayer spacing of the outer three surface layers and of the surface Debye temperature. Our measurement shows a first layer contraction, D₁₂ = −9.0% ± 1.0% and a second layer expansion, D₂₃ = +3.5% ± 1.5% (measured as percent of the bulk interlayer spacing) with a surface Debye temperature of 395 K. The present result is in excellent agreement with two recent LEED measurements.