Adsorbate structure in the H-inducedp4greconstruction of the Cu(001) surface

We have used helium-atom diffraction to study the structure of atomic hydrogen adsorbed, at saturation coverage, on the Cu(001) surface. Helium atoms are more sensitive to the adsorbate structure than low-energy electrons and we observe a (2 {}2 ×{}2 {}2 )R45^∘{} unit cell at saturation in contrast to the structures reported in previous low-energy electron-diffraction (LEED) studies. Our analysis favors a p4g clock rotation of approximately 12^∘{} in the substrate with hydrogen atoms adsorbed in a mixture of fourfold-hollow and shifted bridgelike sites. The adsorbate arrangement has a p2mg symmetry and is consistent with existing LEED, 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×{}1) order, may indicate that a lower coverage may be realized in experiment. The driving mechanism for the substrate clock rotation appears to be the energy gain from adsorption and it can be rationalized by the presence of strong repulsive adsorbate interactions.