Diffraction of correlated electron pairs from crystal surfaces

The spectra of the internal energy distribution of correlated electron pairs, ejected from W(0 0 1), Fe(1 1 0) and Cu(0 0 1) following the impact of a low-energy electron, show characteristic structure that can be associated with the diffraction of two-electron quasi-particles from the periodic surface potential. In this combined theoretical and experimental work we show that the positions of these features in momentum space are determined by the amount of change of the center-of-mass wave vector of the pairs with respect to the surface reciprocal lattice vector. This corresponds to a two-particle Bragg diffraction. The relative intensities of the peaks and the shapes of the peaks depend mainly on the internal correlation of the electron pair, i.e. on the strength of the electronic interaction.