ABINITIO CALCULATIONS OF THE INITIAL-STATE AND FINAL-STATE EFFECTS ON THE SURFACE CORE-LEVEL SHIFT OF TRANSITION-METALS

The surface core-level shift (SCLS) can be obtained from density-functional theory calculations in two quite different ways. First, in an ''initial state'' approximation, the SCLS is taken equal to the difference of the core levels of the surface and bulk-like atoms in a self-consistent calculation for the unperturbed surface. Second, in a more involved ''final state'' theory, the effect of the core hole is included by comparing calculated total energies for a Z + 1 impurity at the surface and in the bulk. The second approach takes into account the possibly different screening of the core hole near the surface, but has the disadvantage that it involves an expensive calculation for a surface impurity. We present calculated SCLS for some typical transition metals using the full-potential LMTO method and both approaches. The results for the smoother surfaces of all 4d transition metals within the initial state picture can be well explained using a standard model based on narrowing of the surface atom density of states and local charge neutrality. The comparison of initial- and final-state calculations is used to draw conclusions about the nature of the screening in d and sp metals.