A LEED-IV investigation of the Ru(001)-p(2 × 1)-H structure

Below 72 K chemisorbed hydrogen on Ru(001) forms three different ordered structures for the coverages θ= 1 3: p(√3×√3), θ= 1 2: p(2×1) and θ= 3 4:(2×2)-3H. For the p(2×1) structure which shows the most intense superstructure reflexes, LEED-IV curves were measured and compared with calculations for different structural models. In agreement with earlier VLEED work we found adsorption on roughly threefold fcc sites, but now a larger vertical distance of 1.34±0.15 Å, together with a buckling and pairing reconstruction of the first Ru layer of 0.02±0.02 Å and 0.04±0.06 Å, respectively. A slight expansion of the first layer distance of the ruthenium substrate of +1%±2%, in contrast to the contraction of -2% observed for clean Ru, is also found. A clear decision for the occupied site was only possible after inclusion of a zero point vibration of 0.4 Å in the calculations. Anharmonicity coupled with this vibrational amplitude is most likely the reason for our relatively large bond lengths. Beyond the specific system, our finding of reconstruction of a close-packed surface by a rather weakly adsorbed atom, and the importance of the zero-point motion of H for the analysis, appear to be of general interest.