Electron momentum spectroscopy of atoms, molecules and solids

Electron Momentum Spectroscopy (EMS) is the application of the (e, 2e) technique to obtain detailed information on the dynamic structure of atoms, molecules, and condensed matter. The ability of EMS to provide wavefunction mapping, in terms of the independent particle representation of a manybody system in momentum space, is discussed for valence electrons. The quantitative measurement of correlation effects in both the initial and final electron states is demonstrated. Correlations can significantly change the momentum density distribution, as well as giving rise to transitions forbidden in the independent particle approximation. The case of argon is discussed in some detail since both initial state correlations (d-wave correlations) and final state correlations (mainly in the 3 s-1 manifold) are present. The influence of relativistic effects in outer valence momentum densities is demonstrated for Xe and Pb. The importance of correlations in the outer valence orbitals of some molecules is discussed with particular reference to H2O. The first (e, 2e) results on an excited target and also an oriented target are discussed, as are some recent high resolution measurements on amorphous carbon. (orig.)