Many-Body Coulomb Systems above the Threshold for Total Break-Up

Abstract

The simplest nontrivial many-body Coulomb problem consists of three charged particles. Situations leading to three charged particles in their continua are (i) double photoionisation of an atom, and (ii) single ionisation of an atom, for instance by an electron. Alternative cross sections may be observed experimentally. In non-coincidence experiments one measures total, singly and doubly differential cross sections. In these cases one obseves only one of the escaping fragments, often an electron. Coincidence experiments, on the other hand, observe two of the fragments. This may be done with or without analysis of the energy distribution between these fragments. The most sensitive probe for a collision model needs comparison with a triply differential cross section, a coincidence measurement including the measurement of the energy distribution. Only in this case the kinematic of the reaction is fully determined. Theoretical work needs then neither averages nor integrations before comparison with the experiment can be made. Such triply differential cross sections (TDCS) have been measuered since 1969 for electron impact ionisation of atoms, socalled (e,2e) measurements, see /1/. Triply differential crosss sections for double photoionisation have been measured only very recently for the first time /2/.