(e,2e) on surfaces

The application of electron-electron coincidence experiments (e,2e) to solid samples has been extensively exploited only during the past few years and the applicability to surfaces has just started to be pursued. The feasibility of (e,2e) experiments in grazing angle reflection geometry has been recently established for the first time. The possibility of using the grazing angle (e,2e) technique as a binding energy and/or momentum spectroscopy of surface states rests on the accurate knowledge of the ionization mechanism. A double collision, elastic and inelastic at large and small momentum transfer respectively, appears to be the dominant process responsible for reflection (e,2e) events. In the present paper, the results of recent (e,2e) experiments performed at energies from thresold up to 300 eV and with kinematics from normal to grazing incidence are reviewed. They allow elucidation of the (e,2e) ionization mechanism on surfaces at both grazing and normal incidence.