Defect surface structures studied by Leed

Low energy electron diffraction has as its primary objective determination of the size, symmetry and contents of the surface unit mesh, but in the case of an adsorbed layer at any coverage less than the maximum the corresponding LEED pattern will contain additional features which are dependent upon the degree of organisation of the adsorbate species. The structure of such a layer must be defective, at very least in the sense that it will not be continuous over the substrate; it may consist, for example, of islands surrounded by clean surface, or the unoccupied sites may be randomly distributed. In addition, other forms of defect will often be present - various adsorbate regions may be displaced relative to one another, a number of different types of unit mesh may occur simultaneously, the adsorbate spacings may not be related to those of the substrate, and so on. The diffraction pattern produced by any of these types of structure will also be defective. Most often this is manifest simply as a high background intensity or a broadening of the spots, but in certain instances split spots, streaks, rings etc. are observed.