Structure and dynamics of clean and adsorbate-covered crystal surfaces studied by surface X-ray diffraction
Applied Physics a-Materials Science & Processing 67(6): 645-656
Surface X-ray diffraction has become an important tool for the study of clean and adsorbate-covered crystal surfaces. We briefly summarize the most important theoretical and experimental aspects and report some recent results. For the adsorption of K on Cu(001) we first demonstrate the determination of the local. K-adsorbate site by analyzing the intensity distribution along the crystal truncation rods. The second example 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 rows and the valley bridge site in the groove between the Si dimers. Actually, the dimerization is lifted upon AM adsorption. Apart from the determination of the atomic coordinates, often also the detailed analysis of the thermal disorder is a prerequisite for the correct surface structure 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 distances determined too short if the anharmonicity is neglected. Finally, some new experimental and theoretical developments in SXRD are presented, 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.