Observation ofd-band narrowing near copper and nickel surfaces by means of angle-resolved x-ray photoelectron spectroscopy

Abstract

Angle-resolved x-ray photoelectron spectroscopy has been utilized to determine the full widths at half maximum intensity (FWHM's) and the second moments of the $d$ bands of Cu and Ni with variable surface sensitivity. At grazing emission for which the mean excitation depth corresponds to only about 1-2 atomic layers, unambiguous decreases of ∼ 10-12% in the FWHM's and ∼ 18-20% in the second moments are observed relative to the bulk-sensitive values obtained for high-angle emission. These decreases are caused by the reduced coordination number at the surface and are consistent with quantitative predictions based on several recent theoretical studies. The experimental data are compared with theoretical spectra obtained from tight-binding densities of states calculated by Kleinman et al. for each atomic layer in a thin metal film. Allowance is made for inelastic scattering of photoelectrons, the refraction of electrons at he surface, and the instrumental resolution. Good agreement is obtained with respect to both the degree of $d$-level narrowing and the changes in fine structure with electronemission angle. The presence of approximately ½ monolayer of adsorbed oxygen is however found to have no significant effect on the surface $d$-band widths.