Autoionization coincidence experiments

Autoionization spectra resulting from specific resonances can be obtained by electron-electron coincidence measurements (Haak et al. 1984 Ungier and Thomas 1983, 1984, 1985). To associate a fr.rgmentation pattern with a particular core hole excited state and a particular autoionization or Auger decay channel, a double-coincidence experiment must be done using electron impact excitation. The energy of the scattered electron must be determined, the energy of the emitted electron must be detennined, and the ions produced in coincidence with these two events must be determined. The difficulties inherent in these kinds of experiments have been aptly summarized by Hitchcock (1989), If you can do it by photons, don t waste your time with electron-coincidence techniques. ... 

By comparing threshold and energetic electron detection in coincidence studies, one can conclude that in the former case a 50-500 times higher signals-to-noise ratio is possible, while in the latter case the photon flux of a Hel radiation source is greater by several orders of magnitude. As a final result, coincidence rates are comparable in the two types of experiment. In practice, the threshold approach seems to be preferred because a wider variety of states is available since autoionization aids to excite levels not available in conventional photoelectron spectroscopy. 

Up to now only a few experiments were performed in which autoionization electrons were analyzed in coincidence with scattered electrons " or scattered ions. " Figure 3 shows an experimental setup as used in our group. Electrons ejected at a variable angle d with respect to the projectile beam direction are analyzed in coincidence with ions, scattered through (6, d>). By fixing the scattering angle 6 and the azimuthal orientation... 

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