The history, development, and future prospects for (e,2e) spectroscopy

Electron momentum spectroscopy of atoms, molecules, and solids is based on (e,2e) reactions that observe the distribution of recoil momenta for energy-resolved states of the residual system. It is interpreted simply in terms of the momentum-space orbitals of the independent-particle model. The relevant ideas originated in nuclear physics. The earliest experiments observed that strongly excited final states belong to orbital manifolds that extend the independent-particle ideas to correlated systems. Some weakly excited final states do not belong to orbital manifolds. They give sensitive information about target ground-state correlations. The energy-momentum distribution of valence bands is observed for solids. Calculations for atoms, molecules, and crystals converge to the experimental result as the structure calculation is improved.