  We report a study of cold charge-transfer (CT) collisions of Rb atoms with
N$_2^+$ and O$_2^+$ ions in the mK regime using a dynamic ion-neutral hybrid
trapping experiment. State- and collision-energy- dependent reaction rate
coefficients have been measured for both systems. We observe markedly different
charge-transfer kinetics and dynamics for the different systems and reaction
channels. While the kinetics in some channels are consistent with classical
capture theory for the rate coefficient, others show distinct non-universal
dynamics. The experimental results are interpreted with the help of classical
capture, quasiclassical trajectory and quantum scattering calculations using
state-of-the-art ab-initio potentials for the highly excited molecular states
involved. The theoretical analysis reveals an intricate interplay between
short- and long-range effects in the different reaction channels which
ultimately determines the CT dynamics and rates. At short range, CT was found
to occur via both single and multiple collision events with the latter
typically showing pronounced large-amplitude internal motions of the collision
complex. Our results illustrate salient mechanisms that determine the
efficiency of cold molecular CT reactions.
