REARRANGEMENT OF STEPPED RU(001) SURFACES UPON OXYGEN-ADSORPTION

The adsorption of oxygen on two different stepped Ru(001) surfaces (n( 001) x 1(010) with n = 13 or 39) was investigated by LEED. For the hig h (low) step density surface, saturation with oxygen leads to a (2 x 2 ) LEED pattern with broadened spots of sixfold symmetry at normal inci dence in the temperature regime between 300 and 500 (750) K. This is d ue to the incoherent superposition of diffraction from the two nonequi valent terrace types separated by monoatomic steps. After annealing to temperatures between 500 (750) K and 1200 K or adsorption at these te mperatures, however, the patterns become threefold symmetric correspon ding to the threefold symmetry of one terrace. Using spot profile anal ysis and the comparison of experimental LEED-IV curves as analytical t ools, we conclude that only one type of terrace, separated by double s teps, is present after annealing, and that the local geometry within t he terraces is the same as on the nominally flat surface in both tempe rature regimes. The predominant terrace type for higher adsorption tem peratures can be identified by comparison of the experimental IV curve s with calculated curves. In doing so, we find that the step face cont ains a fourfold hollow site whose occupation by oxygen is proposed to be the driving force for this surface reconstruction with considerable material transport.