Reflection (e,2e) experiments from surfaces

The application of electron–electron coincidence experiments (e,2e) to solid samples, although begun in the earliest stages of the development of such a spectroscopy, 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. In spite of the success, 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. To understand which is the dominant process responsible for the reflection (e,2e) events, under various energies and kinematical conditions, is still an open problem. Two possible mechanisms are envisaged that can generate pairs of correlated electrons in the reflection geometry: a single inelastic collision at large momentum transfer or a double collision (elastic and inelastic) at high and low momentum transfer, respectively. In the present paper are reviewed the results of recent (e,2e) experiments performed at energies from threshold up to 300 eV and with kinematics from normal to grazing incidence. They allow elucidation of the (e,2e) ionization mechanism on surfaces at both grazing and normal impact.