Role of multi-adatom interactions in the formation of ordered structures on metal surfaces: Application to H/Fe(110)

Abstract

The interaction energy between adatoms on a metal surface is investigated within multiple-scattering theory. Interaction energies for groups of up to six adatoms are calculated and compared to the sum of the corresponding pairwise interactions. The contribution arising from non-pairwise interactions is found to be small but of the same order of magnitude as the long-range pair interactions which are invoked in the formation of ordered overlayers on the surface. Application is made to the H/Fe(110) chemisorption system. Calculation of the one-electron energy change on moving the H atom from a free-electron environment to an Fe(110) environment suggests that short-bridge sites are occupied by the H adatoms. Nearest-neighbour pair interactions are found to be considerably repulsive indicating that H saturation coverages should not be greater than 1. On the basis of calculation of pair interactions only, it is found that at lower H coverages, two ordered structures may be formed, however when including multiple-adatom effects a marginal preference for the experimentally observed c(2 × 2) structure is recorded. Beyond saturation of this ordered overlayer, other ordered overlayers of (3 × 3) structure may be formed, however since more substantial repulsive interactions have to be involved in the formation of such structures one would expect these to be accompanied by a considerable reduction in the hydrogen heat of adsorption.