The coverage-dependent ordering of chemisorbed hydrogen on the (110) surface of nickel

Abstract

The dissociative adsorption of hydrogen on Ni(110) at low temperatures has been studied in great detail with He diffraction. With increasing hydrogen exposure, a c(2 × 6), a c(2 × 4), two different c(2 × 6) and a (2 × 1) phase corresponding to coverages of θ = 1 3 , 1 2 , 2 3 , 5 6 and 1 monolayers (ML) have been identified. The corrugation functions of all phases have been determined by intensity analyses of He-diffraction spectra using the hard-wall approximation. The analyses were performed in a model-free manner using symmetry-compatible Fourier representations of the corrugations, and the results were underpinned by additional calculations with the adatoms modeled as Gaussian hills. The best-fit corrugations of all phases up to 1 ML deliver direct pictures of the adatom configurations, and allow an absolute coverage determination on purely crystallographic grounds in good agreement with the relative coverages determined by flash-desorption measurements. The most important structural elements are long H-zig-zag chains along the close-packed Ni rows with the hydrogen atom sites near threefold coordination. The rather long-ranged lateral interaction between the hydrogen chains favors alternation of zig-zag with zig-zag elements in all submonolayer phases. The c(2 × 6) phase with θ = 5 6 ML constitutes an interesting intermediate as 1 6 ML H-adatoms are obviously forced into twofold coordinated sites to form a distorted close-packed hexagonal pattern of H atoms. Thus, the phase jump of 2.5 Å of all zig-zag chains necessary to form the (2 × 1) where all close-packed Ni rows are covered with parallel zig-zag chains seems to become feasible via a compression of the c(2 × 6) with θ = 5 6 ML . Further hydrogen can subsequently be accommodated via an adsorbate-induced reconstruction of the Ni substrate. Improved diffraction experiments provide the full two-dimensional corrugation function for the final (1 × 2) structure which corresponds to a saturation coverage of ≅ 1.5 ML whereby probably 1 2 ML hydrogen is adsorbed on the second Ni layer.