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Abstract

We review experimental studies of electron impact ionization of atoms and molecules which became feasible by applying the electron-ion momentum vector coincidence technique. Besides briefly discussing excitation of metastable helium we present studies of single ionization of helium and argon and also mention a pilot experiment for positron impact ionization. The main strength of the method is revealed if three or more particles are to be detected and if the cross section of the reaction is small. Exemplarily helium double ionization close to threshold is shown. Studies of molecular ionization reveal a molecular alignment dependent collision dynamics for the molecular hydrogen target. For larger molecules ionized electronic orbital and ionic fragment can be correlated.