Helium-atom diffraction were used to study the structure of atomic hydrogen adsorbed, at saturation coverage, on the Cu(001) surface. Helium atoms were more sensitive to the adsorbate structure than low-energy electrons and a (2√2 x 2√2)R45° unit cell was observed at saturation in contrast to the structures reported in previous low-energy electron-diffraction studies. The analysis favoured a p4g clock rotation of approximately 12° in the substrate with hydrogen atoms adsorbed in a mixture of fourfold-hollow and shifted bridge-like sites. The adsorbate arrangement had a p2mg symmetry and was consistent with existing low-energy electron-diffraction, thermal-desorption spectroscopy and electron-energy-loss spectroscopy results. The ideal structure implies a saturation coverage of 1.25; however, features in the data, including domain boundaries with a local (4 x 1) order, could indicate that a lower coverage could be realized in experiment. The driving mechanism for the substrate clock rotation appeared to be the energy gain from adsorption and it could be rationalized by the presence of strong repulsive adsorbate interactions.

Adsorbate Structure in the H-Induced p4g Reconstruction of the Cu(001) Surface. A.P.Graham, E.M.McCash, W.Allison: Physical Review B, 1995, 51, 5306-10