Rearrangement of stepped Ru(001) surfaces upon oxygen adsorption

The adsorption of oxygen on two different stepped Ru(001) surfaces (n(001) × 1(010) with n = 13 or 39) was investigated by LEED. For the high (low) step density surface, saturation with oxygen leads to a (2×2) LEED pattern with broadened spots of sixfold symmetry at normal incidence in the temperature regime between 300 and 500 (750) K. This is due to the incoherent superposition of diffraction from the two nonequivalent terrace types separated by monoatomic steps. After annealing to temperatures between 500 (750) K and 1200 K or adsorption at these temperatures, however, the patterns become threefold symmetric corresponding to the threefold symmetry of one terrace. Using spot profile analysis and the comparison of experimental LEED-IV curves as analytical tools, we conclude that only one type of terrace, separated by double steps, is present after annealing, and that the local geometry within the terraces is the same as on the nominally flat surface in both temperature regimes. The predominant terrace type for higher adsorption temperatures can be identified by comparison of the experimental IV curves with calculated curves. In doing so, we find that the step face contains a fourfold hollow site whose occupation by oxygen is proposed to be the driving force for this surface reconstruction with considerable material transport.