Orbital Energy Spectra of Electrons in Chemisorption Bonds: O, S, Se on Ni(100)

Abstract

This work presents the first experimental determinations of orbital energy spectra of electrons in chemisorption bonds. Our method, ion neutralization spectroscopy, determines a transition density function which is essentially the local density of states in the surface region of the solid. For the particular surface formed by chemisorption of the chalcogens in ordered, surface-crystalline arrays on Ni(100), the local density of states includes a strong component from the orbital energy spectrum of the electrons in the bonds of the surface molecules formed from the adsorbate atom and Ni atoms presented by the substrate lattice. The orbital levels of electrons in the surface bonds are broadened resonances or virtual states in whose energy range local wavefunction magnitude and hence transition probability for ion neutralization are enhanced. These orbital energy spectra, coupled with measurement of work-function change on adsorption, suggest, in many instances, plausible identifications of local bonding structures. For the specific cases of O, S, or Se(≡X) adsorbed in c(2 × 2) or p(2 × 2) arrays on Ni(100), the following tentative conclusions are drawn. The S or Se adsorbed in the c(2 × 2) structure forms a local bridge-type Ni2X surface molecule in which the chalcogen is bridged across the diagonal of the (1 × 1) surface unit square Ni mesh. The orbital energy spectrum in this case shows three orbital peaks in the available energy range analogous to what is seen for the free H2X molecule having C2υ symmetry. The S or Se in the p(2 × 2) surface arrangement exhibits a simpler spectrum in the available energy range consistent with π-type bonding in a pyramidal Ni4X structure of C4υ local symmetry. Oxygen, on the other hand, in either c(2 × 2) or p(2 × 2) surface-crystal structures, produces a broad resonance whose displacement relative to the energy of the atomic p orbital shows O to be considerably more negatively charged in its structure than are S and Se in theirs. This result coupled with measurements of work-function change and thermal behavior points strongly to a reconstructed surface in which O replaces Ni in the topmost layer of the crystal.