Practical Limitations and Direct Methods

Given the high amount of methodical and computational work necessary for the intensity analysis of complex structures, methods to retrieve the (or some) structural information from the intensities directly, that is, without the trial-and error-based approach, are highly desirable. So, it was possible to use a linearized version of TL to directly retrieve the correct model, yet only when the reference was aheady close enough to it [57, 58]. There are, however, also first promising steps toward real direct methods that work without an a priori assumed model. 

In a rather simplified view, the formation of diffuse intensities can be idealized by the interference of the adatom s direct backscattering and its scattering via the 

As described, the holographictype methods have in common that multiple scattering contributions must be eliminated to some extent. Although the corresponding image reconstruction schemes have been successful in several cases (for reviews, see Ref [31, 32]), it is - at present - not clear how powerful they are in general. 

Elimination of nonkinematic scattering processes is also essential for another direct method, which, however, does not provide the full structural information but 

Pendry, J.B. (1974) Low-Energy Electron Diffraction, Academic Press, London. 

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