Post-collision interaction,

Sch82] Schmidt V 1982 Post-collision interaction in inner-shell ionization, in X-ray and atomic inner-shell physics, ed. B Crasemann, AIP Confer. Proceed. No. 94 (AIP, New York) p. 544. [Sch86] Schmidt V 1986 Z. Physik D - Atoms, Molecules and Clusters 2 275. 

The time between the escape of the electron from the shape resonance, or from the molecule if there is no shape resonance, and Auger decay of the core hole may be sufficiently short to allow the outgoing photoelectron to participate in the Auger process. This participation can be pictured as an interaction between the Auger electron and the photoelectron and is studied under the rubric post-collision interaction (Schmidt 1987). 

After the ejection of an electron from an inner shell, relaxation generally occurs by the emission of a secondary electron. This is known as the Auger effect. It can be reasonably well described as a two-step process, leading to double ionisation, because the primary and Auger electron are usually separate. However, if the initial photoelectron is emitted with a very low kinetic energy, then the Auger electron can catch up and interact with it. This process is described as post-collision interaction or PCI. 

Figure 19. Photoionization map for GeCl4. The inner shell hole states corresponding to atomic 2D5/2 produces the curved but roughly vertical concentrations of intensity for electron energies below 1 eV. A similar line for the 2D3/2 hole is present but hardly visible in the figure. Curvature of the lines is caused by post-collision interaction (PCI).

K. Helenelund, S. Hedman, L. Asphmd, U. Gelius, K. Siegbahn. P. Froelich and O. Goscinski Post-Collision Interaction in the Photo-Excited Electron Spectrum of Selenium (Preprint 1986). 

POST-COLLISION INTERACTION IN INNER-SHELL IONIZATION... 

This phenomenon, called in the literature Post Collision Interaction (PCI), is a strong collective effect, which prominently modifies the photoionization cross section. For not too large and fast Auger electrons it is sufficient to include the interaction of outgoing electron with the rearranged ion field, which is presented by diagrams in Fig. 9. 

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