Ionisation dynamics at intermediate momentum transfer: an (e, 2 e) investigation on argon

Relative triple differential cross section for the coplanar asymmetric (e, 2 e) reaction in argon have been measured at 1.5 KeV incident energy and 40 eV ejected electron energy in several kinematics. Depending on the scattering angle, ?{symbol}a, the chosen kinematics select either ionising collisions belonging to the Bethe ridge (?{symbol}a=9.2°) or processes in the intermediate region between the pure dipolar and binary regimes. The more relevant finding is the presence of a minimum in the recoil lobe, almost opposite to the direction of the momentum transfer. This feature is qualitatively explained by a first Born model, which describes the ejected electron by a Coulomb wave-function. This result suggests that in the investigated kinematics the interaction of the slow ejected electron with the residual ion is the dominant effect beyond the first order electron-electron interactions.