Ion-neutral chemistry at ultralow energies:Dynamics of reactive collisions between laser-cooled Ca$^+$ or Ba$^+$ ions and Rb atoms in an ion-atom hybrid trap*

Cold chemical reactions between laser-cooled Ca$^+$ or Ba$^+$ ions and Rb atoms were studied in an ion-atom hybrid trap [1]. Reaction rate constants were determined in the collision energy range $E_{coll}/k_B =$ 20~mK-20~K. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes including the radiative formation of CaRb$^+$ and BaRb$^+$ molecular ions has been analyzed using accurate potential energy curves and quantum-scattering calculations for the radiative channels. It is shown that the energy dependence of the reaction rates is governed by long-range interactions, while its magnitude is determined by short-range non-adiabatic and radiative couplings. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral collisions.\\[4pt] [1] F.H.J. Hall et al., Phys. Rev. Lett. 107, 243202 (2011).\\[0pt] [2] F.H.J. Hall et al. accepted by Mol. Phys. (arXiv:1301.0724).