  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.
