ADSORPTION STAGES OF O ON RU(0001) STUDIED BY MEANS OF SCANNING-TUNNELING-MICROSCOPY

The adsorption of O on Ru(0001) at a temperature of 400 K is studied i n detail by means of scanning tunnelling microscopy (STM). With increa sing O coverage, an ordered p(2 x 2) structure develops, followed by a p(2 x 1) structure. While the p(2 x 2) structure grows via island for mation, the p(2 x 1) structure is abruptly formed by a disorder-order phase transition. After completion of the p(2 x 2) structure at a cove rage of 0.25 ML, the surface develops a rough structure where the (2 x 2) units remain visible but appear with different heights. As the ori gin 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 x 1) structure are observed. First, small fragments of p(2 x 1) rows devel op, which are randomly distributed over the surface and rotated by 120 degrees with respect to each other. They grow in one dimension and in duce a criss-cross arrangement of linear chains of O atoms. Two-dimens ional ordering starts via pairing of the p(2 x 1) rows. At a critical O coverage slightly below 0.40 ML, suddenly large p(2 x 1) domains are formed in three orientations (rotated by 120 degrees), which coexist with remnants of the p(2 x 2) structure. At the saturation coverage of O (0.5 ML), the p(2 x 1) domains cover the surface completely.