terials like the (1×2) reconstruction of Au(110), the intensity will increase to 106 cps. There are few fields of science where such enormous intensity enhancements have taken place in such a short time. Though few mainstream surface systems have yet been studied with x-ray diffraction, these early experiments have provided valuable symmetry information. Additionally, unique information has been obtained about step densities on surfaces. Structural determinations, however, must rely rather heavily on measurements of the diffracted intensity as a function of momentum transfer. Considering these problems, it is not surprising that the actual atomic positions derived from x-ray data are being refined with other structural techniques, a situation that will change dramatically within a few years.

Results

Surface structural investigations are at a stage where different investigators, in different laboratories, using different techniques, studying the same system, can obtain the same results. The multilayer relaxations on Cu(110) and Ni(110) discussed below have been determined with LEED and with ion scattering, and the two experiments agree with respect to all structural parameters to better than 0.01 angstrom. This length scale is useful for discussing structural changes in chemical reactions in the gas phase. It therefore appears likely that within the next several years we will be investigating the geometrical adsorbate-substrate response to a chemical reaction with the accuracy necessary to have an impact on problems of this kind.

Another class of systems where ion scattering and LEED agree extremely well includes the (110) faces of GaSb, InAs, and GaAs. These surfaces consist of equal numbers of anions and cations located in an ideal crystal in the plane of the surface. It has been known for several years that, at the surface, the anion moves out and the cation moves in. It is now known that the axis joining the two forms an angle of 29 degrees with the surface plane and the agreement between the two techniques is 1 degree or better. The two techniques have different strengths, nevertheless: ion scattering has excellent sensitivity to lateral movements, and LEED is sensitive to normal displacements.

The surface structure of metals was long considered a relatively uninteresting field. Metallic screening at the surface, it was argued, is so short ranged that only minor rearrangements of the atoms would be expected at the surface. This view has changed tremendously over the last few years with the observation of phase transitions on clean metal surfaces as well as those induced by adsorbates. Surface melting has been observed at a temperature far below the bulk melting temperature. Even the structure of ideal surfaces has shown some unexpected features.

It has been found, for example, that on clean metal surfaces the separation



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