Resonant scattering in low energy electron diffraction: Bi/Ni(111)

We report Low Energy Electron Diffraction (LEED) diffraction patterns measured at energies up to 50 eV for a monolayer thick Bi film on Ni(111). Surprisingly, the intensity versus energy profiles of several from the ten unique (i.e., symmetry-independent) sets of spots show finite but pertinent intensity, each only at a well-defined energy. These are attributed to resonant scattering, involving transient capture in eigenstates of the image potential, followed by (multiple) scattering into the vacuum. By its nature, transient capture occurs closely before the energy crosses the Ewald sphere for each considered channel. These energies are one-to-one connected with the corresponding lattice parameters of the Bi-film with its centered rectangular structure, commensurate along Ni[11-2] and high order commensurate along Ni[-110]. In addition, a couple of more intense regular spots show anomalously high intensity at the low energy side upon crossing the Ewald sphere. This feature is attributed to resonant scattering as well. We claim that so far grossly disregarded resonant scattering is a general phenomenon and should be considered in very low energy LEED-IV structural analysis. The intensity versus energy profile of the (0 2) peak does not show obvious evidence for resonant scattering but instead reveals that the Bi film is built up by long (> 20 nm) and narrow (<< 20 nm), translationally shifted domains, oriented along the [-110] azimuth.