Current status of (e,2e) measurements of energy-momentum densities

Electron Momentum Spectroscopy (EMS), or (e,2e) spectroscopy, is based on measuring the relative differential cross sections for kinematically complete high energy, high momentum transfer, electron impact ionization events. Under these EMS conditions the ejected electron is knocked cleanly out of the target. Knowing the energies and momenta of the incident and two emitted electrons one can infer the separation (or binding) energies and momenta of the ejected electrons within target or sample. The momentum profile of the differential cross section is directly related to the momentum probability distribution of the Dyson orbital, i.e. square of the (quasi) particle amplitude obtained by overlapping the N electron target with the N-1 electron ion state. Transitions to different ion states can give detailed information on initial as well as final state correlation effects. For condensed matter targets it amounts to a measurement of the quasi-particle energy-momentum density of occupied bands. The principles of the measurement and its application to atoms, molecules, and amorphous, polycrystalline, and single crystal materials are discussed