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Content archived on 2024-04-16



Direct ionisation studies including the measurement of accurate absolute cross sections for the triple differential (e,2e) ionisation cross section (TDCS) near to threshold, at intermediate energies and at very high energies have permitted quantitative comparisons with theories, some of which differ by large factors but have very similar form. The extension of measurements to noncoplanar scattering geometries has allowed detailed tests to theory, including separation of the contributions from singlet and triplet scattering. The important role of double scattering in the electron impact ionisation reaction, in which the incident electron scatters first from the nucleus and then from a target electron, which had been theoretically predicted, was experimentally investigated over a wide range of kinematics and energies and very accurately calculated. At relativistic energies, spin flip processes have been discovered to be important, in contrast to expectations. Cooperation between experimentalists and theoreticians is finally making clear the mechanisms for ionisation near to threshold, where various high energy approximations are inappropriate and the full 3-body coulomb problem must be considered.

Measurement of cross sections for ionisation in which the ion is left in an excited state have tested the theoretical understanding of correlated 2-electron processes. Also the effects of shape resonances on (e,2e) cross sections in molecules have been detected and measured for the first time.

The investigation of photon or electron impact double ionisation phenomena has also taken great steps forward, experimentally and theoretically and absolute 5-fold differential cross sections for double ionisation (ie involving triple coincidence experiments) have been measured for the first time.

The effect of a strong laser field on the target (dressed states) and the ionisation reaction has been calculated in detail and is predicted to result in important measurable effects , opening up a new subfield for study.

Furthermore, a great deal of effort has been devoted to the development of multicoincidence, multiparameter data collection systems in order to increase the signal rates by an order of magnitude or more and hence permit inner shell and triple coincidence experiments which are presently infeasible, thus opening up new fields of investigation. The application of the (e,2e) method to the measurement of electronic states in solids is also being studied.
Extension of the former activities realized by french groups (RCP n. 784 -C.N.R.S.) during 1984-87. Realize and develop a structured collaboration with various exchanges and cooperations between several european groups involved in (e, 2e) coincidence techniques and their applications to physical and chemical studies (electronic subshell studies of atoms or molecules in gas phase, clusters, thin films, surfaces...). Contributions to plasma problems and ionisation mechanisms (single, multiple or laser-assisted) as complementary to photoionisation techniques.


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