Coherences between autoionising states of different excitation energies

The authors have observed coherences between the electron impact excitation of autoionising states of helium that are separated in energy up to more than 2 eV. The method is based on the occurrences of interferences between scattered and ejected electrons resulting from the electron impact excitation and subsequent autoionisation of the states concerned. No energy shifting or broadening mechanisms such as those caused by post-collision interactions play a role in the experiment. The authors also present a theoretical description of the observed coherence effects. From this theory a parametrisation for scattered- and ejected-electron profiles is deduced which is fitted to experimental data. This parametrisation appears to describe the observed state-state interference very well.