The adsorption of O on Ru(0001) at a temperature of 400 K is studied in detail by means of scanning tunnelling microscopy (STM). With increasing O coverage, an ordered p(2 × 2) structure develops, followed by a p(2 × 1) structure. While the p(2 × 2) structure grows via island formation, the p(2 × 1) structure is abruptly formed by a disorder-order phase transition. After completion of the p(2 × 2) structure at a coverage of 0.25 ML, the surface develops a rough structure where the (2 × 2) units remain visible but appear with different heights. As the origin of this phenomenon, we propose additional O-O interactions and/or subsurface O due to the increase in O coverage. At coverages between 0.3 monolayer (ML) and 0.35 ML, different preformations of the p(2 × 1) structure are observed. First, small fragments of p(2 × 1) rows develop, which are randomly distributed over the surface and rotated by 120 • with respect to each other. They grow in one dimension and induce a criss-cross arrangement of linear chains of O atoms. Two-dimensional ordering starts via pairing of the p(2 × 1) rows. At a critical O coverage slightly below 0.40 ML, suddenly large p(2 × 1) domains are formed in three orientations (rotated by 120 • ), which coexist with remnants of the p(2 × 2) structure. At the saturation coverage of O (0.5 ML), the p(2 × 1) domains cover the surface completely.

The development of the morphology of an Ag(100) single‐crystal surface bombarded with 600 eV Ar+ ions at 170 K and at room temperature is studied by spot profile analysis of LEED. A temperature‐dependent saturation of the step density is observed and a distinct smoothing of the surface after bombardment occurs already at room temperature. Under out‐of‐phase condition the LEED spots show a fourfold shape that differs in orientation at both temperatures. Monte Carlo simulations of the atom removal including thermal surface diffusion reveal at 170 K the formation of 〈100〉 and 〈110〉 step edges with equal probability, whereas at room temperature rearrangement processes at the steps lead to the preferential formation of the close‐packed 〈110〉 edges. The intensity distribution under out‐of‐phase condition calculated from the Monte Carlo snap shots exhibits the same temperature dependence of the spot shapes as observed experimentally. The interlayer mass transport occurring during annealing at room temperature is found to be based on jumps running downward the 〈100〉 step edges.

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