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

TL;DR

This study investigates cold ion-neutral reactions between laser-cooled Ca^+ ions and Rb atoms at ultralow energies, revealing the energy dependence of reaction rates and quantum resonances using experimental and theoretical methods.

Contribution

It provides new experimental data and quantum-chemical analysis of reaction dynamics at ultralow energies in an ion-atom hybrid trap.

Findings

Reaction rate constants follow the Langevin model at low energies.

Short-range couplings influence the magnitude of reaction rates.

Narrow shape resonances are predicted at specific collision energies.

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 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 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.