We analyze reactive collisions of a single ${\mathrm{Ba}}^{+}$ ion in contact with an ultracold gas of Rb atoms at low three-body collision energies of 2.2(9) $\mathrm{mK}\times k_{\mathrm{B}}$. Mapping out the ${\mathrm{Ba}}^{+}$ loss rate dependence on the Rb atom density we can discern two-body reactive collisions from three-body ones and determine both rate coefficients, which are $k_2=3.1\left(6\right)\left(6\right)\times {10}^{-13}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$ and $k_3=1.04\left(4\right)\left(45\right)\times {10}^{-24}{\mathrm{cm}}^6{\mathrm{s}}^{-1}$, respectively (statistical and systematic errors in parentheses). Thus, the measured ternary recombination dominates over binary reactions even at moderate atom densities of $n\approx {10}^{12}{\mathrm{cm}}^{-3}$. The results for ${\mathrm{Ba}}^{+}$ and Rb are representative for a wide range of cold ion-atom systems and can serve as guidance for the future development of the field of hybrid atom-ion research.

• Received 3 February 2016

DOI:https://doi.org/10.1103/PhysRevA.94.030701