Energy scaling of cold atom-atom-ion three-body recombination

We study three-body recombination of Ba${}^{+}$ + Rb + Rb in the mK regime where a single ${}^{138}$Ba${}^{+}$ ion in a Paul trap is immersed into a cloud of ultracold ${}^{87}$Rb atoms. We measure the energy dependence of the three-body rate coefficient $k_3$ and compare the results to the theoretical prediction, $k_3\propto E_{\text{col}}^{-3\mathrm{/}4}$ where $E_{\text{col}}$ is the collision energy. We find agreement if we assume that the non-thermal ion energy distribution is determined by at least two different micro-motion induced energy scales. Furthermore, using classical trajectory calculations we predict how the median binding energy of the formed molecules scales with the collision energy. Our studies give new insights into the kinetics of an ion immersed into an ultracold atom cloud and yield important prospects for atom-ion experiments targeting the s-wave regime.