T.K. Sham

Effect of the Madelung potential on surface core-level shifts in GaAs☆

Abstract It is shown that the difference between the Madelung potentials at the (110) surface and in the bulk of GaAs is the major factor determining the core level shifts which have recently been observed and that site charge differences between surface and bulk, if significant, are such that Ga and As are less ionic at the surface than in the interior. These considerations have general relevance.

X-ray absorption studies of unoccupied d-band states in gold and nickel metallic compounds

Abstract Au L 2,3 edge XANES of Au 2 Ag, AuAl 2 , and AuGa 2 and Ni L 2,3 edge XANES of NiAl and NiSb have been analyzed. Emphasis was placed on the derivation of unoccupied d-state population from the integrated absorption coefficient coefficient (area) under the white line. The results correlate very well with those obtained previously from Mossbauer and X-ray photoelectron measurements, hence confirming the validity of the d-non-d charge compensation model. The Ni results show that the d-non-d interplay is less dramatic in Ni metallic compounds.

Ionic charge reduction at the surfaces of crystals

Abstract Because the atomic constituents of a compound have lower coordination in the surface of a solid than in the interior one expects that compounds are less ionic at crystal surfaces than in the bulk, e.g. that cations at surfaces are in lower states of oxidation. A general expression is obtained here for a bound on such charge reduction at nonpolar surfaces. This shows that surface atoms, as a rule, have their ionic charge reduced relative to the value in the bulk by at most a factor of two.

Photoemission Studies Of Carbon Overlayers On Nb

We have studied the electronic properties of the amorphous, graphitic, and carbidic phases of carbon layers on niobium. While features of the valence hand photoemission and absorption cross sec-tion spectra have been discussed in separate publications, this presentation focusses on the observation of resonant photoemisston and the photon-induced Auger spectra.