The influence of the Frenkel excitons in molecular crystals on the elastic differential scattering cross section for the low-energy-electron diffraction

In this paper a low-energy-electron diffraction theory for molecular crystals is presented. The possibility of creating Frenkel excitons by the incident low-energy electrons is taken into account. The evaluation of the terms of the differential scattering cross section is based on a set of diagram rules, which has been discussed in previous papers. The sum of the contributions of the elastic diagrams with one wavy line is now evaluated in the dipole approximation. The result will be used in a series of subsequent papers on the influence of the bulk and surface excitons on the LEED intensities.