Electron momentum spectroscopy of atoms, molecules and thin films

The theory of electron momentum spectroscopy (EMS), a technique based on th e single (multiple) electron impact ionization of a quantum target (atom, m olecule, thin Nm), is reviewed. The advantages of EMS as a means for interp reting outgoing momentum distributions as compared to other methods for dir ectly measuring the many-body wave function of the target are vastly illust rated. It is shown that EMS contributes profoundly to advances in the quant um chemistry of molecules allowing, in particular, to gain insight into how electron orbitals transform as molecules approach one another adiabaticall y, and to study in detail the band structure and one-electron wave function s in both crystalline and amorphous thin films. The possibility of the dire ct double ionization "portraying" of the electron correlations is also stud ied.