Cold chemical reactions between laser-cooled Ca+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies E coll /kB = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca + ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb + molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb + and quantumscattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by shortrange non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. 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 reactive collisions.

The investigation of cold interactions between ions and neutrals has recently emerged as a new scientific frontier at the interface of physics and chemistry. Here, we report a study of charge-transfer (CT) collisions of Rb atoms with N and O ions in the mK regime using a dynamic ion-neutral hybrid trapping experiment. We observe markedly different CT kinetics and dynamics for the different systems and reaction channels studied. While the kinetics in some channels are consistent with classical capture theory, others show distinct non-universal dynamics. The experimental results are interpreted with the help of classical-capture, quasiclassical-trajectory and quantum-scattering calculations using 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. Our results illustrate salient mechanisms that determine the efficiency of cold molecular CT reactions.

We present a dynamic ion-atom hybrid trap for studies of cold ion-neutral collisions and reactions with a significantly improved energy resolution compared to previous experiments. Our approach is based on pushing a cloud of laser-cooled Rb atoms through a stationary Coulomb crystal of cold ions using precisely controlled, tunable radiation-pressure forces. We demonstrate the tuning of the atom kinetic energies over an interval ranging from 30 mK up to 350 mK with energy spreads as low as 24 mK inferred from the comparison of experimental time-of-flight measurements with Monte Carlo trajectory simulations. We also demonstrate first applications of our method to the investigation of chemical reactions. Our development opens up perspectives for accurate studies of the energy dependence of the reaction rates, the dynamics and the reaction-product ratios of ion-neutral processes in the cold regime. It also paves the way for the realisation of fully energy-and state-controlled cold-collision experiments.

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