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) struct ure analysis for the clean as well as hydrogen-saturated (110) surface s of tungsten and molybdenum. Both clean surfaces exhibit some contrac tion of the first interlayer spacing, i.e. -3.1% for W and -4.0% for M o, whereas deeper layer distances remain practically bulklike. The onl y structural consequence of a hydrogen monolayer adsorbed is the reduc tion of the first interlayer distance contraction to approximately hal f 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 hyd rogen-induced lateral shift of the top layer of W(110). Hydrogen itsel f is found to be adsorbed in threefold-coordinated hollow sites at a h eight of about 1.2 Angstrom and 1.1 Angstrom above the first substrate layer of W and Mo, respectively.