Rydberg Redivivus in Surface Physics

Beam threshold effects have been observed in low-energy electron diffraction from a very early date. These effects were discovered as fine structures in the intensity of the specularly reflected beam. They occur in a 1–2 eV range below the energy of grazing emergence of a new beam. I 1977 it became clear that at least some of the fine structures were induced by the image potential barrier in front of a metal surface. The coulombic potential on one side of the surface plane and the metal reflectivity on the other act as a two-dimensional waveguide which can hold metastable states arranged in a Rydberg series of spectral constant 1/16 Ryd. At present up to three levels of the series can be disentangled by a high-resolution LEED apparatus.