Berakdar, J.

The simultaneous emission of two electrons from condensed matter following the absorption of a linearly polarized photon is studied in the first order-perturbation theory for the radiation field, and within the dipole approximation. The double emission from localized and delocalized electronic states is considered. It is shown that spectra of the emitted pairs obey propensity rules expressed by the scalar product of the center-of-mass vector momentum of the pair and the photon's polarization vector. Furthermore, it is shown that diffraction of the pair from the lattice occurs when the pair's center-of-mass wave vector changes by a lattice reciprocal vector during the photoemission. For semi-infinite solids with delocalized valence electrons, an initial state is constructed from single-particle orbitals. Following the absorption of the photon the valence-band electron pair propagates into the vacuum within the screened electron-electron Coulomb potential. Numerical results for a clean Cu crystal are analyzed for photon polarization parallel and perpendicular to the surface.

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