  We report on a study of cold reactive collisions between
sympathetically-cooled molecular ions and laser-cooled atoms in an ion-atom
hybrid trap. Chemical reactions were studied at average collision energies
<Ecoll>/k > 20 mK, about two orders of magnitude lower than has been achieved
in previous experiments with molecular ions. Choosing N2+ +Rb as a prototypical
system, we find that the reaction rate is independent of the collision energy,
but strongly dependent on the internal state of Rb. Highly efficient charge
exchange about four times faster than the Langevin rate was observed with Rb in
the excited (5p) 2P3/2 state. This observation is rationalized in terms of a
capture process dominated by the charge- quadrupole interaction and a near
resonance between the entrance and exit channels of the reaction. Our results
provide a test of classical models for reactions of molecular ions at the
lowest energies reached thus far.
