A refined LEED structural determination for the surface designated Ni (111)–(2 × 2)–S

Abstract

A new intensity analysis with low-energy electron diffraction is reported for the (2 × 2) surface structure obtained by the adsorption of H2S on the (111) surface of nickel. Intensity-versus-energy curves were measured with a video LEED analyzer for 10 diffracted beams at normal incidence, and comparisons were made with intensity curves calculated with multiple-scattering methods for models in which S atoms chemisorb at three-fold coordinated adsorption sites, but with the possibilities of both lateral and vertical relaxations in the local metallic structure. Small adsorbate-induced relaxations are found, but the dominant structural feature is that the S atoms adsorb above the "expected" adsorption sites (i.e. those which continue the regular fee packing) with a 1.50 Å spacing between the S layer and the top-most Ni layer. The S–Ni bond length of 2.10 Å agrees to within 0.02 Å of a prediction using a current bond length – bond order relation, but this value is smaller than two other recent measurements by SEXAFS and ion scattering by 0.06 and 0.10 Å, respectively. This analysis also finds the first two Ni layer spacings are expanded from the bulk value by 2 to 3%. Keywords: LEED, surface structure, S chemisorption, Ni(III) surface.