Abstract
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.
