D. Hennig

Ab initio study of hydrogen adsorption on Pd(100)

Abstract We report an all-electron density-functional theory study of the adsorption of hydrogen at Pd(100). We use the local-density approximation for the exchange-correlation energy functional and the full-potential linear muffintin orbital method (FP-LMTO) to calculate adsorption energies, stable adsorption sites, adsorption-induced surface relaxations, and the work-function changes. It is found that (as expected) for coverages θ ⩽ 1 the surface fourfold hollow site has the largest adsorption energy. For coverages θ > 1 it is predicted that additional hydrogen is incorporated below the surface. The work function increases with hydrogen coverage up to θ = 1 but for additional hydrogen adsorption we find that ΔΦ remains roughly constant. The theoretical results are compared with available experimental data.

Adlayer core-level shifts of admetal monolayers on transition metal substrates and their relation to the surface chemical reactivity

Using density-functional theory, we study the electronic and structural properties of a monolayer of Cu on the fcc(100) and (111) surfaces of the late 4d transition metals, as well as a monolayer of Pd on Mo bcc(110). We calculate the ground states of these systems, as well as the difference of the ionization energies of an adlayer core electron and a core electron of the clean surface of the adlayer metal. The theoretical results are compared to available experimental data and discussed in a simple physical picture; it is shown why and how adlayer core-level binding energy shifts can be used to deduce information on the adlayers chemical reactivity. \textcopyright{} 1996 The American Physical Society.

Calculated surface energies of the 4d transition metals: A study of bond-cutting models

The surface energies of the low-index surfaces of the 4d transition metals were calculated using density-functional theory and the full-potential linear muffin-tin orbital method. The results are discussed in the context of bound-cutting models. It is found that the usual approach, which ignores the decrease of the bond strength with increasing coordination number, is incompatible with the calculated results. The coordination number — bond strength relation was calculated explicitly for several metals, showing that the bond strength decreases roughly as the reciprocal of the square-root of the coordination number. A modified model which takes this into account correctly describes the magnitude and the surface depencence of the calculated surface energies. In addition, we discuss the previously ignored role of the free-atom orbital structure in bond-cutting models.

Ab-initio calculations of the initial- and final-state effects on the surface core-level shift of transition metals

Abstract 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.

Enhanced screening of core holes at transition-metal surfaces

Ab initio calculations based on density-functional theory were used to obtain surface core-level shifts for the 4d transition metals and silver in the initial-state model and in the full-impurity formulation, giving an unambiguous separation into initial state and screening terms. This shows that the screening of the core hole is substantially better at the surface than in the bulk for a transition metal. For Ag, an opposite and even larger effect is found, showing the central role of d-electron screening in the surface core-level shift of the transition metals.

Ab initio calculation of surface core-level shifts for transition metal surfaces

The surface core-level shift (SCLS) for a number of smooth surfaces of 4d-transition metals Mo, Rh, Pd and Ag was calculated within two different approaches using the full-potential LMTO method. The first approach, the initial state approximation, estimates the SCLS from the position of the core eigenvalues of atoms at the surface relative to those of the bulk. The second approach treats a surface or bulk atom with a core hole as an impurity by means of a supercell. This approach is in principle exact within the local-density approximation and thus includes final-state screening effects. The results show that the screening of the core hole is of different nature for Ag when compared to the transition metals.

Theoretical approach to isoelectronic δ-doping of ale-structures

Abstract The electronic structure of various arrangements of Te δ-doping layers in ZnSe is calculated using a tight-binding model. Based on the numerical results the photoluminescence data are discussed.