Ion-neutral chemistry at ultralow energies: dynamics of reactive collisions between laser-cooled Ca⁺ ions and Rb atoms in an ion-atom hybrid trap^†

^†Dedicated to Prof. Martin Quack on the occasion of his 65th birthday.

Abstract

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⟩/k _B=20 mK-20 K. The lowest energies were achieved in experiments using single localised 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 analysed using high-level quantum-chemical calculations of the potential energy curves of CaRb⁺ and quantum-scattering 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 short-range 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.

Keywords:

• cold ion-neutral reactions
• hybrid traps
• cold chemistry
• radiative association
• charge transfer

Acknowledgements

The authors acknowledge support from the Swiss National Science Foundation (grants nr. PP0022_118921 and PP00P2_140834) and the COST Action MP1001 ‘Ion Traps for Tomorrow’s Applications’. The authors thank Dr Alexander Gingell for making available his programme for the 8-level OBE treatment of Ca⁺.

Notes

^†Dedicated to Prof. Martin Quack on the occasion of his 65th birthday.