A diagram formalism for the study of the influence of Frenkel excitons in molecular crystals on the low-energy-electron diffraction

In this paper a low-energy-electron diffraction (LEED) theory for molecular crystals is presented. The possibility of creating Frenkel excitons by the incident low-energy electrons is considered. The evaluation of the terms of the differential scattering cross section series is based on a set of diagram rules established in a previous paper. This diagram formalism is now studied in detail. The results will be used in a series of subsequent papers on the influence of the bulk and surface excitons on the LEED intensities.