Reflection (e,2e) experiments: A novel surface spectroscopy

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 ionisation mechanism and on the capability of achieving a sufficiently good energy resolution. 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 plus inelastic). In this paper are presented the results of new (e,2e) experiments that allow to elucidate the ionisation mechanism at intermediate energies (300 eV) and asymmetric kinematics. The measurements, performed on highly oriented pyrolitic graphite, also show that an overall energy resolution as good as 1.2 eV can be achieved.