Leed structure analyses of the clean and fully hydrogen-covered W(110) and Mo(110) surfaces

We present a comparative low energy electron diffraction (LEED) structure analysis for the clean as well as hydrogen-saturated (110) surfaces of tungsten and molybdenum. Both clean surfaces exhibit some contraction of the first interlayer spacing, i.e. -3.1% for W and -4.0% for Mo, whereas deeper layer distances remain practically bulklike. The only structural consequence of a hydrogen monolayer adsorbed is the reduction of the first interlayer distance contraction to approximately half its value of the clean surface, i.e, to -1.7% for W and -2.0% for Mo. Our results give no evidence for an adsorbate-induced reconstruction of either surface and thus discard the widely accepted model of a hydrogen-induced lateral shift of the top layer of W(110). Hydrogen itself is found to be adsorbed in threefold-coordinated hollow sites at a height of about 1.2 Angstrom and 1.1 Angstrom above the first substrate layer of W and Mo, respectively.