Tensor leed for the geometrical and chemical structure of alloy surfaces

Low energy electron diffraction is still the most frequently used technique to retrieve the structure of surfaces. Multiple scattering of electrons on the one hand provides high structural precision but on the other hand also complicates the structural analysis and limits the complexity of accessible structures. This particularly applies to alloy surfaces with their enlarged variety of possible structures which comes by the increased size of the surface unit cell (chemically ordered alloys) or the unknown layer-dependent stoichiometry (disordered alloys). A way out of that dilemma comes by application and further development of the perturbation method Tensor LEED. We show that it can be extended to treat efficiently and accurately the chemical substitution of atoms. This allows easy access to the layer-dependent stoichiometry of chemically disordered alloys. Also, by periodic substitution of atoms, intensities calculated for an elemental crystal can be perturbed to yield the intensities for an ordered alloy. Moreover, we propose and test the application of a direct method which gives the deviations from bulk stoichiometry directly, i.e. without the usual trial and error procedure.