We have systematically studied the electron pair emission from a Cu(100) surface over a wide range of primary electron energies. We are able to identify contributions from valence states of different orbital characters. The relative intensity from states near the Fermi level ($E_F$) and from the $3d$ levels is strongly affected by the primary energy. The primary energy dependence of the pair intensity resembles the behavior of the specularly reflected primary electron beam. We discuss these findings within a kinematical diffraction model. The azimuthal orientation of the sample has a strong effect on the coincidence energy spectra. We performed measurements along two high-symmetry directions and the difference of the energy spectra reveals intensity variations up to 50$\%$. This is an immediate consequence of the symmetry of the surface. This, in turn, proves that the depletion zone in the pair emission is nonspherical.

• Received 3 November 2011

DOI:https://doi.org/10.1103/PhysRevB.85.195120