Helium diffraction from Ni(100): A study of the clean surface and of hydrogen and carbon adsorption phases
He scattering from the clean Ni(100) surface gives extremely weak diffraction beams, corresponding to a very small corrugation amplitude of ≅ 0.01 Å. Hydrogen adsorption at temperatures between 100 and 200 K leads to the formation of an ordered (1× 1) phase with a corrugation amplitude only three times larger than that of the clean surface. Surface charge-density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens from the topmost Ni layer dn ≅ 0.9-1.0 Å. He diffraction from the c(2 × 2) structure of carbon on Ni(100) confirms the p4g symmetry of this phase, and the best-fit corrugation function reflects the different local distortions of the substrate atoms around the corner and centre carbon atoms. Charge-density calculations yield for the carbon atoms in fourfold hollows dn ≅ 0.1-0.2 Å, in good agreement with earlier LEED results.