Surface Science

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

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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. © 1985.


01 Dec 1985


Surface Science