The adsorption of hydrogen on a cobalt(lOio) surface was investigated in ultrahigh vacuum (UHV) between 85 and 500 K using Video-LEED, temperature-programmed thermal desorption (TPD), work function (il) measurements, and high-resolution electron energy loss spectroscopy (HREELS). Between 90 and 200 K, hydrogen adsorbs dissociatively with high sticking coefficient (s 0~0 .8) via precursor kinetics and forms, with increasing exposure, a c(2X4), a p2mg (2X 1) and a (1 X2) LEED structure (hydrogen coverages ®H=0.5, 1.0, and 1.5, respectively). While the first two structures represent true ordered hydrogen phases there is strong evidence that the (1 X2) phase is reconstructed, likely in a paired-row configuration. The formation of the (1 X2) phase is slightly thermally activated; its decomposition produces a sharp thermal desorption maximum (a state) appearing on the low-energy side of a {3-TPD signal which reflects the hydrogen desorbing from the unreconstructed surface. The activation energies for desorption from the a and f3 states are 62 and 80 kJ/mol, respectively. Chemisorption in the f3 state [(2X 1) phase up to ®H= 1.0] is associated with a il of +207 meV, while the fully developed (1X2) reconstructed phase (a state) causes a il of approximately-122 meV resulting in an overall work function change of +85 meV at saturation. From HREELS, we determine the H adsorption site in all superstructures to be threefold with a local Cs symmetry. Our results are discussed and compared with previous findings for similar metalhydrogen interaction systems.

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Co(lOTO) with close-packed rows separated by the oaxis distance is a hep analog to fee (110). In contrast to the (2x1) missing-row (MR) structures formed upon oxygen adsorption on several fee (110) surfaces, a novel type of MR reconstruction is revealed by scanning tunnel microscopy and low energy electron diffraction on Co(lOTO): The low coverage (2x1) O phase constitutes a double-layer MR structure with (2*1) MR configurations in the two topmost Co layers; it develops locally by moving every other atom of the topmost close-packed Co rows into respective short bridge sites of the layer underneath. Results on the c(2x4) and the high coverage (2x1) oxygen phases support this conclusion.

The structure of the clean Co(1010) surface has been analysed by LEED. Application of a recently developed computational scheme reveals the prevalence of the termination A in which the two topmost layers exhibit a narrow spacing of 0.62 A, corresponding to a 12.8( + 0.5)% contraction with respect to the bulk value, while the spacing between the second and third layer is slightly expanded by 0.8( + 0.2)%.The (1010) surface of a hcp crystal represents the counterpart to the well-known (110) plane of fcc crystals. In contrast to the latter, however, so far only one structural analysis (for Re (1010)) has been reported in the literature [1]. As a peculiarity, this kind of surface may exhibit two types of termination as illustrated in fig. 1. The coordination of the topmost atoms differs for the two terminations. In termination A each atom of the top layer is surrounded by four nearest neighbours in the second layer with a relatively small layer spacing. Modifaction B shows a surface atom surrounded by only two nearest neighbours in the second layer with much larger spacing between the two topmost layers. Termination B obviously exhibits a much stronger surface corrugation, and pronounced relaxation effects in the topmost layers are most likely. The differing corrugations of the two surface terminations provide quite different adsorption sites.In contrast to other transition metals relatively few investigations have dealt with cobalt single crystal surfaces. Concerning the hexagonal Co phase LEED investigations were reported only for the (0001) [2][3][4] and for the (1120) surface [5]. In this work we present a LEED structure analysis of the clean Co(1010) surface. The analysis was carried out applying the automatic optimization scheme and the rDE-factor developed by Kleinle et al. [6], final results were cross-checked with standard r-factor methods.The experiments were carded out in a standard UHV chamber at a base pressure p < 10-8pa. Details about the experimental setup are described elsewhere [7]. The cobalt single crystal with a size of 6 mm diameter and 1 mm thickness was orientated within 0.5 ° in the [1010] direction by Lamprecht GmbH, D-7531 Neuhausen. After mechanical polishing the crystal was electrochemically etched as described by Welz et al. [5] resulting in a mirror-like finish. Thereafter the crystal was demagnetized by a Helmholtz coil, spotwelded between two tantalum wires and attached to a sample manipulator. The crystal was adjusted on the sample manipulator in a way to allow normal incidence within 0.5 o. The crystal temper-0039-6028/91/$03.50 © 1991 -Elsevier Science Publishers B.V. (North-Holland)