Low‐Energy Electron Diffraction (LEED)

Abstract

Principles and HistoryWhen Davisson and Germer reported, in 1927, that the elastic scatting of lowenergy electrons from well -ordered surfaces leads to diffraction spots similar to those observed in X -ray diffraction [1 -3] where h is Planck ' s constant, m e the electron mass, v the velocity, and E kin the kinetic energy of the electron. Already, Davisson and Germer realized that the diffraction of low -energy electrons (also termed low -energy electron diffraction ; LEED ) could be used to determine the structure of single crystal surfaces, in analogy to X -ray diffraction, if their kinetic energy was between 40 and 500 eV -that is, if their wavelength ranged between 0.5 and 2 Å . Due to their small inelastic mean free path ( IMFP ) of only a few Å ngstroms (typically < 10 Å ), electrons in this energy range sample only the topmost atomic layers of a surface and are, therefore, better suited for the analysis of surface geometries than X -ray photons, which have a much larger mean free path (typically of a few micrometers). However, unlike for photon diffraction, multiple scattering plays an important role in the diffraction process of electrons at solid surfaces. Therefore, the analysis of LEED data with respect to the exact positions of atoms at the surface is somewhat more complicated, and requires fully dynamical quantum mechanical scattering calculations. The use of LEED for surface analysis became important when suffi ciently large single crystals became available for surface studies. Initially, the technique was used only for the qualitative characterization of surface ordering and quantitative determination of the two -dimensional ( 2 -D ) surface lattice parameters (e.g., superstructures, see below). Information regarding the positions of the atoms in the