Case studies in surface geometry: Adsorbate and coadsorbate structures on Ru(001), and some implications

Using a CCD LEED system for the collection of IV data with minimal beam damage, and full dynamical as well as Tensor LEED calculations, the geometries of about 20 adsorbate and coadsorbate structures on Ru(001) have been determined over the past decade. With work by other groups, more than 30 accurate adsorbate structures are now available for this surface. The advantage of this concentrated effort directed at a single surface with equivalent methods is that comparisons of trends can be made on this basis, and that the correlation of different structures lends additional credibility to the separately determined structural parameters. This then justifies comparisons with results of first principles theoretical calculations and other conclusions. Here a short overview is given of the methods used and of the structures derived, and some methodological questions (reliability of "LEED only," and the importance of supporting evidence; reproducibility vs. accuracy; temperature effects and the influence of soft vibrations) are discussed. This data set is then used to briefly discuss some examples, including: • Influences of site selection and environment on the adsorbate structure; • Lateral interactions in coadsorbate structures; • Possible chemical contributions in rare gas adsorption; • Global and local relaxations of the Ru surface atoms and their interpretation. Comparison to theoretical results is made where possible.