Oxygen induced reconstruction of a close-packed surface: A LEED IV study on Ru(001)-p(2 × 1)O

A LEED IV study was carried out on the apparent (2 × 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 LEED calculations are: The (2 × 2) structure at half monolayer coverage is due to incoherent mixing of 3 domains of a p(2 × 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 × 1) unit cell. This result is the first direct evidence that also close-packed surfaces can be reconstructed by strongly chemisorbed adlayers. Although amplitudes of atomic shifts are below 0.15 Å 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 1.2 ± 0.02 Å. Vertically, both first and second Ru layers stay at their bulk positions, but are buckled in addition. The changes in effective Ru radii are attributed to charge redistribution between Ru surface atoms and oxygen.