LEED structure analysis of the clean and (2×1)H covered Pd(110) surface

Abstract

From an analysis of the low-energy electron diffraction (LEED) intensities we have determined the oscillatory distortion of the topmost interlayer spacings of the clean and the (2×1)H covered Pd(110) surface as well as the exact adsorption geometry in this latter structure, which is formed at T<180 K at a coverage of θH≤1.0. An R-factor analysis was used for quantitative comparison with the experimental data. The oscillatory distortion of the clean surface— d12=−5.1±1.5%, d23=+2.9±1.5%, d34 at its bulk value of 1.37 Å (with R factors RP=0.22 , RZJ=0.14 )— is found to be reduced by the H adlayer to d12=−2.2±1.5%, d23=+2.9±1.5%, and d34=bulk value. The H atoms are adsorbed on quasi-threefold sites with equal distances of 2.0±0.1 Å to the two nearest Pd neighbors in the topmost and the closest Pd atom in the second layer, leading to an effective radius of the H atom of rH=0.6±0.1 Å. The long-bridge adsorption site, (octahedral) subsurface sites, or a hydrogen induced reconstruction via a lateral displacement of topmost Pd atoms by more than ±0.1 Å can clearly be ruled out. These structural data, which are in good agreement with those of ordered Had structures on Ni(110) and Fe(110), characterize the (2×1)H structure on Pd(110) as being a typical adsorbate structure. There is no indication of either direct occupation of distinct subsurface or of near surface adsorption sites in this structure, nor does it open up channels for surface penetration, e.g., by a strong distortion of the topmost Pd substrate layers. In contrast to Ni(110) and Rh(110) the island growth of the (2×1)H structure on Pd(110) indicates predominantly attractive, indirect adatom–adatom interaction which, however, are of distinctly different nature than those causing island formation in the Pd–hydride phase.