Selection rules in (e,2e) spectroscopy from surfaces

For coplanar geometries of two-electron emission from non-magnetic crystalline surfaces due to the collision of an incident low-energy electron with a valence electron, the spatial symmetry of the latter entails a selection rule for the total scattering cross section, which vanishes for states antisymmetric with respect to the reaction plane, and another one for the partial scattering cross sections for detecting pairs with equal and with opposite spins. These selection rules are strictly valid in the absence of spin-orbit coupling and approximately so in its presence. The general analytical results are specialised to the Cu(110) surface, for which we also present numerically calculated (e,2e) spectra together with the symmetry-resolved bulk band structure and layer density of states.