DIFFUSE LOW-ENERGY-ELECTRON DIFFRACTION

Up to the mid-1980's low energy electron diffraction (LEED) was consid ered as a surface structural tool applying only to surfaces with long- range order and exhibiting sharp diffraction patterns. It was then poi nted out first by theoretical work that long-range order is not really essential: diffuse LEED (DLEED) intensities, particularly those appea ring in disordered adsorption on a crystalline substrate, carry inform ation about the local adsorption structure. Different approaches have been proposed and put into practice to calculate the diffuse intensity distribution. Experimental techniques to measure and process the diff use data were developed based on conventional LEED optics, on a channe l plate design and on energy dispersive detectors. A number of disorde red structures have been solved as reviewed in this paper. The interpr etation of diffuse intensities also yielded a new understanding of con ventional LEED: Both DLEED and LEED intensities are created by the loc al scattering object and it is only interference between ordered or di sordered adsorption clusters which forms sharp spots or diffuse intens ity distributions. This local picture of electron diffraction also led to a holographic interpretation of diffuse intensity distributions al lowing reconstruction of real space atomic images in favourable cases.