(e,2e) collisions at intermediate energies

Recent experimental results will be reviewed for the atomic (e,2e) ionization process at incident energies in the `low energy' region, from a few eV to about 100 eV above threshold. This region is particularly interesting because it embodies all the possible complexities that can be present in the electron-impact ionization process, such as distortions, multiple scattering, correlations, post-collisional interactions and exchange effects. The helium (e,2e) experimental results presented recently by the Manchester group have been obtained over an exceptionally wide range of scattering angles, and have provided the first opportunity to derive multipole angular correlation information in a direct way. This type of analysis has been applied to measurements symmetric in both scattering angle and outgoing electron energy obtained at incident energies from 10 to 50 eV above threshold. A significant result is that the lower incident energies give rise to the highest multipole components. This is a clear and direct indication that the profound effects known to be produced by electron-electron interaction in the threshold region continue to exist in the low energy region.