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 developme nt 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,?e) 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 s urface states rests on the accurate knowledge of the ionization mechan ism. To understand which is the dominant process responsible for the r eflection (e,2e) events, under various energies and kinematical condit ions, is still an open problem. Two possible mechanisms are envisaged that can generate pairs of correlated electrons in the reflection geom etry: a single inelastic collision at large momentum transfer or a dou ble collision (elastic and inelastic) at high and low momentum transfe r, respectively. in the present paper are reviewed the results of rece nt (e,2e) experiments performed at energies from threshold up to 300 e V and with kinematics from normal to grazing incidence. They allow elu cidation of the (e,2e) ionization mechanism on surfaces at both grazin g and normal impact.