IONIZATION IN ION-ATOM COLLISIONS - OFF-SHELL AND 3-BODY EFFECTS

The recently developed channel-distorted 2-centre (D2C) approximation remains to be evaluated in exact form but two different approximation schemes that lead to computable forms have been identified. In one of these schemes the ionization amplitude is expressed as a product of th ree factors with distinct characteristics. The first term is the off-e nergy-shell amplitude for electron scattering in the field of the appr oaching ion and represents a pure projectile property. The second term is the inelastic form factor and represents a pure target property. T he third term represents a residue of a renormalization factor due to long-range and off-shell aspects of the considered interactions and is manifestly of three-body character. The second scheme is more accurat e than the first but less transparent from a physical point of view an d restricted by additional conditions on the form of the channel poten tial. It is shown that final-state interaction and channel distortion are very essential elements of the theory and various characteristic f eatures in the ionization spectrum are discussed. Finally, we identify the popular CDW-EIS theory as a special case of the D2C theory and so lve an outstanding question about the applicability of the CDW formula tion.