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

Electron pair emission from ferromagnetic crystalline surfaces, which occurs due to the collision of an incident low-energy electron with a valence electron, is shown to be governed by spatial and spin selection rules. Valence electron states with spatial parts antisymmetric with respect to the reaction plane are not observable for any spin configuration. If there is a spatial symmetry plane normal to the reaction plane, spin-polarized primary electrons selectively probe equal and opposite spin valence electrons. These analytical findings are further elaborated for ferromagnetic cubic (110) surfaces and illustrated by numerical results for Fe(110)