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The value of the low-energy electron diffraction (LEED) technique for the evaluation of surface ordering depends on the ability to measure the intensity profiles of diffraction beams with respect to the associated surface component of the electron momentum transfer. Beam profiles, if measured with sufficient accuracy, may be interpreted to characterise the extent of surface order (e.g. distribution of step spacings) and to differentiate between different modes of disordering (e.g. surface melting versus roughening). The ability to measure LEED intensity profiles has been enhanced by use of low-current well-defined primary electron beams in conjunction with position-sensitive detection (PSD) of the diffracted electrons. The following are examples of applications ofLEED-PSD. Compositional Ordering at Ordering Alloy CU3Au (100) and (110) Surfaces: The ordering of the (100) surface is .believed to conform to a conventional picture in which the already-ordered bulk acts as a template, but the profiles measured in the course of ordering of the (110) surface are of the shapes expected if the ordering occurred first at the surface. Disordering of Ce(111) Surface 150 K below the Bulk Melting Temperature: The intensities and profiles are inconsistent with surface .melting or roughening, but a model based on molecular dynamics simulations is not ruled out. Order and Disordering at Decagonal Quasicrystal AI65 CUI 5 C02 0 Surfaces: At room temperature the quasi crystalline order is well developed at both the 'ten-fold' surface (perpendicular to the ten-fold surface (perpendicular to the ten-fold periodic axis) and a 'two-fold' one (parallel to the ten-fold axis) as evidenced by narrow beam profiles. The ten-fold surface undergoes a disordering transition at 700 K, but the temperature dependence of the profiles is unlike that expected for the roughening transition anticipated theoretically.

Applications of Low-energy Electron Diffraction to Ordering at Crystal and Quasicrystal Surfaces