  We have studied the associative ionization of a Rydberg atom and a ground
state atom in an ultracold Rydberg gas. The measured scattering cross section
is three orders of magnitude larger than the geometrical size of the produced
molecule. This giant enhancement of the reaction kinetics is due to an
efficient directed mass transport which is mediated by the Rydberg electron. We
also find that the total inelastic scattering cross section is given by the
geometrical size of the Rydberg electron's wavefunction.
