Electron-electron coincidences experiments on surfaces, status and perspective of a new spectroscopy

Abstract
The past decade has witnessed a sizeable increase of activity in the field of electron-electron coincidence on surfaces. Electron collision and photon absorption have been exploited to perform correlated experiments in a variety of kinematics and involving valence as well as core electrons. In these break-up experiments the kinematics of the ionising event is fully determined. In this talk I shall concentrate on those experiments performed in reflection geometry as they are thought to be particularly suited to investigate surface rather than bulk properties. To understand which is the dominant mechanism that leads to ejection of pairs of electron from the surface and what novel information is yielded by these spectroscopies is the main aim of this work. This issue will be discussed in two cases : a) the grazing and normal incidence (e,2e) experiments performed at low and intermediate energies, b) the first angle resolved Auger-photoelectron coincidence experiment. It will he shown that for both classes of experiments, depending on the kinematics chosen, the correlated behaviour of the pairs of electrons detected might prevail on the independent particle one