The role of Frenkel excitons in low-energy-electron diffraction theory for molecular crystals

In this paper a low-energy-electron diffraction theory for molecular crystals is established. The possibility of creating Frenkel excitons by the incident low-energy electrons is taken into account. The whole formalism is based on a set of diagram rules, which facilitates the evaluation of the differential scattering cross section. The influence of the bulk and surface excitons on the LEED intensities will be discussed in a series of subsequent papers.