Orbital Energy Spectra Research Articles

Orbital identification in the INS and UPS spectra of chalcogens adsorbed on Ni(100)

Orbital energy spectra of the c (2×2)Te structure on Ni(100) obtained by both UPS and INS show two Te-derived orbital peaks which we interpret as arising from the nonbonding and bonding orbitals of the surface Ni2Te molecular complex. These conclusions are suggested by the comparison of orbital peak shapes with those of the nonbonding 5d orbitals of Hg adsorbed on Ni(100) and by the energy positions relative to the p4 orbital energy level in free Te. Previously unpublished data on c (2×2)Se as well as considerations of relaxation energy indicate that this interpretation should also be carried over to the INS spectra for c (2×2)Se and c (2×2)S reported previously. Relative orbital intensity in the INS and UPS spectra support this interpretation if the chalcogen atom in the c (2×2) structure is bridge-bonded across the diagonal of the basic surface Ni square as previously proposed. INS and UPS spectra for c (2×4)Te show a single broad orbital peak requiring a different bonding configuration, probably of C4v symmetry involving four Ni atoms.

Some Topics Concerning the Electronic Characterization of Solid Surfaces

Two electronic spectroscopies of solid surfaces, ion-neutralization spectroscopy and ultraviolet photoemission spectroscopy, are discussed with particular reference to their joint use in determining orbital energy spectra of electrons in surface adsorption complexes. The goal is the exploitation of the differences in matrix element and surface sensitivity when the two spectroscopies are applied to the same surface. Localization of surface orbitals and factors determining the orbital energies are discussed.

Orbital energy spectra of CO and Hg adsorbed on Ni(100): G E Becker and H D Hagstrum (Proc 19th Nat Symp Am Vac Soc), J Vac Sci Technol, 10(1), Jan/Feb 1973, 31–34

Orbital energy spectra of CO and Hg adsorbed on Ni(100): G E Becker and H D Hagstrum (Proc 19th Nat Symp Am Vac Soc), J Vac Sci Technol, 10(1), Jan/Feb 1973, 31–34

Orbital Energy Spectra of CO and Hg Adsorbed on Ni(100)

Orbital energy spectra of CO and Hg adsorbed individually and co-adsorbed on Ni (100) have been determined by ion-neutralization spectroscopy (INS) and ultraviolet photoelectron spectroscopy (UPS) at 45 ° incidence. In the case of CO, two orbitals derived from the 5σ and 1π orbitals of free CO are observed 7.8 and 11.1 eV below the Fermi level, respectively, by UPS, with INS revealing only the 7.8-eV orbital since 11.1 eV is outside its accessible range. In the case of Hg, UPS and INS both reveal orbital peaks 7.8 and 9.7 eV below EF, identified, respectively, with the 5d5/2 and 5d3/2 states of Hg. The Hg(6s) orbital is not in evidence in the energy spectra obtained by either method. We have determined energy shifts of the observed orbitals with respect to their positions in the free molecule or atom and have discussed these shifts in terms of the effects of bonding and electric charge shifts in the metal-adsorbate complexes. This work permits a fairly detailed intercomparison of INS and UPS. It also bears on the problem of peak shifts due to interference between bulk and surface photoemission and possible perturbation by the probing ion in the case of ion neutralization.

Orbital energy spectra of electrons in chemisorption bonds: O, S, Se ON Ni(110) and Ni(111)

In an extension of previous work on Ni(100), the method of ion-neutralization spectroscopy has been used to determine orbital energy spectra of electrons in the chemisorption bonds holding O, S, or Se in ordered arrays on the Ni(110) and Ni(111) faces. Observations of surface crystalline structure by LEED and of thermal stability properties of the structures formed were made. Work-function changes on adsorption were also measured. These results are used in an attempt to assess the nature of local bonding structures. It is concluded for Ni(110), as for Ni(100), that the evidence points strongly toward reconstruction for the O structures, but not for the S and Se structures. The evidence is less convincing for Ni(111), and no conclusion about reconstruction is drawn. Comparisons between Ni(100) and Ni(110) indicate that when the structure is non-reconstructed the chalcogen bonds to as many Ni atoms as possible, that is, produces pyramidal rather than diatomic or “sidebridge” surface molecules.

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

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.

Surface Molecular Structure from Ion-Neutralization Spectroscopy of Electrons in Surface Orbitals

It is shown that the ion-neutralization spectroscopy determines the orbital energy spectra of surface molecules formed of adsorbed foreign atoms and surface atoms of the substrate. For free molecules such a spectrum yields information concerning molecular symmetry and structure as well as ionicity within the molecule. Using also measured work-function change in adsorption, specific conclusions are drawn concerning O, S, and Se adsorbed in $p(2\ifmmode\times\else\texttimes\fi{}2)$ and $c(2\ifmmode\times\else\texttimes\fi{}2)$ arrangements on Ni (100).