The atomic geometry of c (2×2) overlayers of sulfur on Ni(100)

Abstract

Our earlier determination of the structure of Ni(100)−c (2×2)−S overlayers via the analysis of elastic low−energy electron diffraction (ELEED) intensities is reexamined following a −2.5−eV contact−potential correction to the energies of the ELEED intensities measured by Theeten. While this correction does not alter our conclusions that the surface is unreconstructed and that the S atoms occupy sites of four−fold coordination, it does render dS = 1.3 Å a slightly better choice for the vertical height of the sulfur overlayer than the value of 1.7 Å previously reported. A discussion is given of the influence of the model of the electron−ion−core potential on the ELEED lineshapes and the empirical optical potential parameters. For our overlapping atomic potentials, the use of about ten phase−shifts is required for complete convergence of the single−atom scattering factors over the energy range 100 eV?E?200 eV. Estimation of the optical potential for clean Ni suggests that calculations based on a 10−phase−shift model would exhibit empirical real inner potentials 2−5 eV smaller in magnitude than those obtained in our present 4−phase−shift analysis.