The resolving power of a low-energy electron diffractometer and the analysis of surface defects

The angular distribution of intensity in a LEED diffraction spot gives information on deviations from long-range order in surfaces and overlayers. Such information is in part obscured by the finite resolution of the instrument. It is demonstrated that the instrument can detect phase correlations over distances on the surface far greater than the conventionally used “transfer width” but that this capability has little to do with the ability to resolve deviations from surface order. A quantity, “the minimum angle of resolution”, is defined to describe the resolving power of a LEED instrument. It depends, in addition to the shape and width of the instrument response function, on the accuracy of the measurement. This concept is applied to a variety of model surfaces with varying degrees of short-range order. It is demonstrated that the size of ordered domain that can be resolved depends on the type of defects present. Comparison is made with the traditional “transfer width” approach.