A low-energy electron diffraction study was carried out on the apparent (2 x 2) structure of dissociated oxygen on the hexagonally close-packed Ru(001) surface at saturation coverage. Four non-equivalent integral order beams and six non-equivalent half order beams were measured at normal incidence. Main results of the comparison with dynamical low-energy electron diffraction calculations are: The (2 x 2) structure at half monolayer coverage was due to incoherent mixing of 3 domains of a p(2 x 1) structure. Oxygen chemisorption causes buckling and pairing in at least the first two substrate layers parallel to the mirror plane left for the (2 x 1) unit cell. This result was the first direct evidence that also close-packed surfaces could be reconstructed by strongly chemisorbed adlayers. Although amplitudes of atomic shifts were below 0.15Å both in lateral and vertical directions, both vertical and even lateral shifts significantly improve the agreement of fits. The oxygen atom still occupies a site close to the hcp threefold coordinated site of the unreconstructed surface, at a vertical distance to the outermost Ru layer of 1.2Å. Vertically, both first and second Ru layers stay at their bulk positions, but were buckled in addition. The changes in effective Ru radii were attributed to charge redistribution between Ru surface atoms and oxygen.

Oxygen Induced Reconstruction of a Close-Packed Surface - A LEED IV Study on Ru(001)-p(2 x 1)O. H.Pfnür, G.Held, M.Lindroos, D.Menzel: Surface Science, 1989, 220[1], 43-58