Tamm surface resonances in very low energy electron scattering from   clean metal surfaces
  Very low-energy features which occur in electron reflectivities from clean fcc metal (111) surfaces have been subject to a number of interpretations. Here we analyse the feature near 19.8 eV on Cu(111) at normal incidence and find that it is due to resonant scattering at the rise of the muffin-tin average interstitial potential between atomic layers on approach to the surface from the bulk. This new mechanism corresponds to a Tamm-type surface resonance which is very different in formation to the usual Shockley and Rydberg resonances and explains all features in a systematic way. 
M. N. Read