We report on the observation of an elementary exchange process in an
optically trapped ultracold sample of atoms and Feshbach molecules. We can
magnetically control the energetic nature of the process and tune it from
endoergic to exoergic, enabling the observation of a pronounced threshold
behavior. In contrast to relaxation to more deeply bound molecular states, the
exchange process does not lead to trap loss. We find excellent agreement
between our experimental observations and calculations based on the solutions
of three-body Schr\"odinger equation in the adiabatic hyperspherical
representation. The high efficiency of the exchange process is explained by the
halo character of both the initial and final molecular states.Comment: 4 pages, 4 figure
