Ion-neutral chemical reactions between ultracold localized ions and neutral molecules with single-particle resolution

Chemical reactions between ultracold $^{9}\mathrm{Be}^{+}$ ions and room-temperature molecular hydrogen isotopomers and between ultracold ${\mathrm{H}}_{3}^{+}$ ions and room-temperature ${\mathrm{O}}_{2}$ have been studied in a laser-cooling ion trap apparatus. For small Coulomb crystals of beryllium ions, reactions can be followed at the single-ion level. We demonstrate characterization of a chemical reaction in which neither one of the reactants nor the product is directly detectable. In this case molecular dynamics simulations were used for the determination of ion numbers from images of the $^{9}\mathrm{Be}^{+}$ ion ensemble. The observed reaction rates are in agreement with the Langevin ion-neutral reaction theory.