Electron angular distributions in low-energy (e,2e) reactions

The fully differential cross section for the electron-impact ionization of atomic hydrogen is analysed at an incident energy of 27.2 eV. The cross sec tions are calculated using a variety of theoretical models whose prediction s are compared. It is seen that, as at higher impact energies, the angular distribution basically consists of two peaks which, at first sight, can be assigned to binary and recoil processes. However we show that the similarit y is somewhat illusory in that the shape and position of the two peaks depe nd sensitively on the detection geometry and the energy sharing between the two electrons. Furthermore, the final-state electron-electron repulsion an d the effects of exchange symmetry contribute significantly to the form of the observed angular distributions.