  We report on the study of state-changing collisions between Rydberg atoms and
ground state atoms. We employ high-resolution momentum spectroscopy to identify
the final states. In contrast to previous studies, we find that the outcome of
such collisions is not limited to a single hydrogenic manifold. We observe a
redistribution of population over a wide range of final states. We also find
that even the decay to states with the same $l$ as the initial state, but
different principal quantum number $n$ is possible. We model the underlying
physical process in the framework of a short-lived Rydberg quasi-molecular
complex, where a charge exchange process gives rise to an oscillating electric
field that causes transitions within the Rydberg manifold. The distribution of
final states shows a diffusive-like behavior.
