STRUCTURE AND DYNAMICS OF CLEAN AND ADSORBATE-COVERED CRYSTAL-SURFACES STUDIED BY SURFACE X-RAY-DIFFRACTION

Surface X-ray diffraction has become an important tool for the study o f clean and adsorbate-covered crystal surfaces. We briefly summarize t he most important theoretical and experimental aspects and report some recent results. For the adsorption of K on Cu(001) we first demonstra te the determination of the local. K-adsorbate site by analyzing the i ntensity distribution along the crystal truncation rods. The second ex ample presents some results of the structure analyses on ordered K/Si( 001)-(2 x 1) and Cs/Si(001)-(2 x 1) superstructures. The alkali-metal (AM) adsorption sites are the dimer bridge site on top of the dimer ro ws and the valley bridge site in the groove between the Si dimers. Act ually, the dimerization is lifted upon AM adsorption. Apart from the d etermination of the atomic coordinates, often also the detailed analys is of the thermal disorder is a prerequisite for the correct surface s tructure analysis. This is demonstrated for the (2 x 1)-superstructure induced by room-temperature adsorption of K on Ag(001). Anharmonicity is observed for the top-layer Ag vibrations which leads to Ag-Ag dist ances determined too short if the anharmonicity is neglected. Finally, some new experimental and theoretical developments in SXRD are presen ted, namely surface magnetic scattering, the analysis of large organic superstructures as well as improvements in data analysis using direct methods such as for example the maximum entropy method.