Multiple-excitation pathways in a four-charged-particle system: A Green-function analysis - art. no. 012706

When a charged particle interacts with an atom it may induce double electro nic transitions. The complete information on such a reaction is encompassed in the Green function of the interacting four-particle system (the externa l charged particle, the two electrons, and the residual ion). In this work we employ a perturbation expansion of the Green operator and derive from th at a multiple-scattering series for the scattering operator. As pointed out here, each term in this series corresponds to a particular sequence of bin ary potential collisions. From a simple analytical analysis we identify the regions of the four-body spectrum where the effects of the multiple-scatte ring terms become prevalent. We uncover the existence of a left-right asymm etry in a two-particle collision. This dichroic feature occurs because the symmetry of space is broken by the presence of the particles not participat ing in the two-body encounter. We study the ionization-ionization-transfer reaction when the external charged particle is heavy. The present formal, e xact analysis predicts, in addition to the Thomas peaks, a triple-star peak structure in the spectrum when considered as a function of the momenta of the ionized electron, the residual ion and the scattered projectile. It is shown that the star is two dimensional and that its shape is dependent on t he velocity of the continuum electron. In addition we consider the ionizati on and positronium formation following the scattering of a positron from an atom. We point out that certain terms in the multiple-scattering series co incide due to the equal masses of the electron and the positron which open the way for interference effects.