Origin of the Surface Sensitivity in Surface X-Ray Diffraction

The first half of this paper explains how X-ray diffraction can be sensitive to surface structure and morphology, even though X-rays interact only weakly with matter and hence penetrate deeply into the bulk medium. The basis of the crystal truncation rod construction is given, which demonstrates this sensitivity in a formal way. This is then illustrated with details of two problems of current interest which have been studied with synchrotron radiation at the National Synchrotron Light Source in New York. The structure of the Si(111)7×7 reconstructed surface, as determined in the vertical direction by X-ray reflectivity, is presented as a straightforward application of crystal truncation rods. Then we discuss alkali adsorption on Ag(110) surfaces which induces a "missing row" reconstruction. We measured the trends in the induced structural parameters as a function of Cs coverage on Ag(110), but found the greatest changes were associated with the location of the Cs instead. At high coverage this is ordered only in a one-dimensional sense, but as the coverage is reduced it becomes partially registered in the second direction, and, surprisingly, occupies a site higher above the surface.