Mapping of the electronic structure of surfaces by low-energy (e,2e) spectroscopy

Two-electron emission due to the collision of a low-energy electron with a valence electron of the surface system is formulated on the basis of screened Coulomb matrix elements, which involve the valence electron spectral function. Numerical calculations for the (111) surface of Cu demonstrate that the (e,2e) spectra are closely related to the densities of states of the two top-most atomic layers. In particular, the sp-like Shockley surface state and its dispersion relation E(k∥) can be directly seen in (e,2e) spectra obtained in a coplanar symmetric geometry.