Two-step-wise interpretation of highly asymmetric, grazing angle (e,2e) on solids: A real momentum spectroscopy for surfaces and overlayers

Abstract

This paper deals with the mechanism of grazing incidence (e,2e) events from surfaces. Two different approaches are considered. In both cases, elastic scattering with the crystal lattice assists the inelastic collision; these two steps are coupled either coherently or incoherently. Experimental evidence is given that the "coherent" approach reproduces better the cross section dependence on momentum transfer in the specific case of asymmetric kinematics at moderate electron energies. This model has allowed us to map out the band dispersion of the outermost valence states of highly oriented pyrolytic graphite and to measure the momentum distribution of pi-electron states without invoking the contribution of reciprocal lattice vectors in the momentum conservation. Agreement between theory and experiment is satisfactory, though the presence of events where crystal momentum is reconstructed cannot be ruled out. These results, obtained with a significant reduction of the experiment duration by an implemented apparatus, show that reflection (e,2e) can be used to build up a momentum spectroscopy with high surface sensitivity

Role of surface and bulk plasmon decay in secondary electron emission

“…The kinematics in our experiment is also compatible with an impact ionization process, 31 where an incident electron transfers part of its energy and momentum to a bound electron that is emitted. The plasmon creation and subsequent decay can be interpreted as a resonant channel for the direct impact ionization.…”

Secondary Electron Emission by Plasmon Induced Symmetry Breaking in Highly Oriented Pyrolitic Graphite (HOPG)

“…For sufficiently large losses, this electron may overcome the surface potential barrier and is ejected from the surface. This deflection-loss (DL) model [8,[44][45][46] implies that the scattering kinematics of the inelastic collision are fully determined. In other words, under such conditions, the final state of the correlated electron pair in an (e,2e) experiment is fixed by the energy and direction of detection and together with the known state of the primary electron, the initial state of the ejected solid state electron is fully determined.…”

“…When recording the spectrum of correlated electrons in a Bragg maximum, those processes dominate in which the primary electron is first deflected along the outgoing Bragg beam followed by the inelastic process. In the deflection-loss (DL) model, one thus assumes an initial momentum of the primary electron determined by the Bragg condition [8,[44][45][46]. As a consequence, all initial and final states of the inelastic process are fixed by momentum and energy conservation [40,47].…”

“…In the deflection-loss (DL) model, one thus assumes an initial momentum of the primary electron determined by the Bragg condition [8,[44][45][46]. As a consequence, all initial and final states of the inelastic process are fixed by momentum and energy conservation [40,47]. In other words, the coincidence experiment makes it possible to pinpoint the electronic transition of the bound electron involved in the (e,2e)process by measuring time correlated electron pair intensities.…”