Abstract: SCF‐Xα‐SW ionization energies for chemisorbed chalcogens on Ni(001)

Abstract

The ionization energies for a series of chalcogen–nickel clusters were calculated by the self‐consistent‐field Xα scattered‐wave (SCF‐Xα‐SW) method.1 In accord with the most recent theoretical interpretations2,3 of low‐energy electron diffraction (LEED) data, the chalcogen atoms were assumed to be located in a pyramidal position above a cluster of five nickel atoms whose geometry simulated the (001) surface of nickel. It was assumed that each surface Ni atom contributed to bonding with exactly one chalcogen atom. To consider the resulting four‐fold‐site symmetry found in p (2×2) coverages, C4v symmetry functions were employed; to consider the resulting two‐fold‐site symmetry found in c (2×2) coverages, C2v symmetry functions were employed. Thus, by using the same cluster, two different physical coverages were considered by merely changing the symmetry of the basic functions employed.The lone‐pair electrons which occur on the chalcogen atoms in these very localized bonding descriptions were not properly treated due to the muffin‐tin (MT) averaging used in the calculations. In analogy with SCF‐Xα‐SW calculations on H2O,4,5 overlapping of the MT spheres5,6 was employed to improve the consideration of these lone pairs. Significant improvement resulted due to the greater effective directionality obtained.Since the chalocogens are more electronegative than the nickel atoms, charge is removed from the five nickel atoms and is transferred to the chalcogen. In an actual metal the same transfer occurs, except that in the metal the source of the electrons is not confined to just the metal atoms surrounding the chemisorbed species.7 That is, the metal atoms throughout the solid contribute a small electronic charge to the chemisorbed species. To compensate for this, calculations were also performed in which electronic charge was added to the cluster until the core levels were approximately equal to those of an isolated Ni5 cluster.Further variations in the spacings of the atoms allowed within the error limits of the LEED calculations were considered.8 Specifically, vertical displacements of the chalcogens and horizontal displacements of the atoms involved in bonding for two‐fold‐site symmetry were studied.The ionization energies obtained for the various calculations are compared to the INS (ion neutralization spectroscopy) spectra of Hagstrum and Becker for chalcogens on Ni(001).9 Their surface molecule interpretation of their spectra for chemisorbed chalcogens is discussed in view of our calculations. The importance of relating to a physical picture in order to interpret the results of SCF‐Xα‐SW calculations is stressed and illustrated by example.