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Few-particle quantum-dynamics in ion-atom collisions


Fischer,  Daniel
Daniel Fischer - Emmy Noether Junior Research Group, Junior Research Groups, MPI for Nuclear Physics, Max Planck Society;

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Fischer, D. (2014). Few-particle quantum-dynamics in ion-atom collisions. Habilitation Thesis, Ruprecht-Karls-Universität, Heidelberg.

Cite as: http://hdl.handle.net/11858/00-001M-0000-0024-3D6B-C
The description of the dynamics of more than two mutually interacting particles is one of the most fundamental and, at the same time, most challenging tasks in physics. That is because the equations of motion cannot be solved analytically, a fact which is well known as the “few-body problem”. Inelastic atomic scattering processes in atomic collisions are ideally suited to study the dynamics of such correlated few-particle systems, because here the interaction between the particles – the electro-magnetic force – is well understood. Moreover, experimental techniques are available which, on the one hand, allow preparing the initial states with high precision, and, on the other hand, enable determining the final state momentum balance. From theoretical side, only about 15 years ago the first methods (e.g. the exterior complex scaling method, ECS) were reported that succeeded to describe the simplest atomic three-body systems numerically exact. However, these methods were developed for electron or photon impact and are not applicable to heavy projectiles due to the very different convergence behavior, e.g. in a partial wave expansion. Already one of the simplest imaginable inelastic ionatom collision processes, the single ionization of a hydrogen atom, cannot be described without approximations. Here most theorists have resorted to perturbative methods, such as the First Born Approximation or more sophisticated continuum distorted wave models (e.g. continuum distorted wave – eikonal initial state, CDW-EIS). While for single ionization some features, such as e.g. the electron emission characteristics, are well reproduced by perturbative methods, there are yet unsolved disagreements between experiment and theory in the description of the full three-particle momentum balance. Such discrepancies have been observed even at very low perturbations (the perturbation parameter is η=Z/v, with projectile charge Z and velocity v), where these approximations were expected to describe the collision dynamics well. For higher perturbations or if more particles are involved the theoretical description gets increasingly challenging resulting in an incomplete understanding of the underlying dynamics. The studies described in this summary were performed within a research project funded through the Emmy-Noether program of the German research council (Deutsche Forschungsgemeinschaft, DFG). The aim of this project was the experimental exploration of the dynamics of inelastic reactions in ion-atom collisions. In a first series of experiments, the role of electronic correlation [1-5] and projectile decoherence [6,7] in atomic breakup processes has been investigated with a helium target using a conventional Reaction Microscope. In a second step, a novel experimental tool was developed that allows obtaining data of unmatched quality and detail. This apparatus consists of a laser-cooled and trapped lithium target (MOT) combined with a momentum imaging spectrometer (Reaction Microscope) in a so-called MOTReMi [8]. This device allowed, for the first time, to study the initial state dependence of ion-atom collision dynamics [9,10]. In the following a brief overview of the experimental technique and the most important results is given. For detailed discussions the reader is referred to the corresponding publications.