Observation of Chemical Reactions between a Trapped Ion and Ultracold Feshbach Dimers

We measure chemical reactions between a single trapped ¹⁷⁴Yb⁺ ion and ultracold Li₂ dimers. This produces LiYb⁺ molecular ions that we detect via mass spectrometry. We explain the reaction rates by modeling the dimer density as a function of the magnetic field and obtain excellent agreement when we assume the reaction to follow the Langevin rate. Our results present a novel approach towards the creation of cold molecular ions and point to the exploration of ultracold chemistry in ion molecule collisions. What is more, with a detection sensitivity below molecule densities of 10¹⁴ m^-3, we provide a new method to detect low-density molecular gases.