Postcollision Interaction PCI in Ion-Atom Collisions

This effect was first discussed by Barker and Berry in connection with autoionization electron spectra from He -He collision. In a potential 

For collisions with highly charged ions this effect becomes even more pronounced. Strongly PCI-influenced line shapes with widths on the order of 1 eV were observed, for example, in energy spectra of autoionization electrons from Ne, excited by Ne -H2 collisions. In this case an 

Here A (, p) represents the transition matrix element given in Eq. (13), which can be written as 

With a complex potential = eg i72t for the excited state (to allow for the decay) and a pure Coulombic potential (e) = e + X/R for the final state, Eq. (18) can be evaluated by means of the stationary phase method, which yields 

There is a strong analogy between these interference structures and the so-called quantum beats that are observed in photon emission studies when atoms are coherently excited into different states by beam-foil, laser excitation, or other methods. Both phenomena are due to time-dependent interferences of transitions from different states. In quantum-beat studies 

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