Formation of translationally cold $\mathrmMgH^+$ and $\mathrmMgD^+$ molecules in an ion trap

We have produced and cooled the molecular ions MgH+ and MgD+ in a linear Paul trap. These ions were generated by the photochemical reactions Mg+(3p2P3/2)+H2 (D2)→MgH+ (MgD+)+H (D), and identified by the radial separation in the trap of ions with different charge-to-mass ratios. The molecular translational motion was cooled sympathetically by Coulomb interaction with laser-cooled Mg+ ions to a temperature estimated to be below 100 mK. Ordered structures (ion crystals) containing more than 1000 ions, with more than 95% being molecular ions, were obtained. Such translationally cold and well-localized samples of molecular ions could become very useful for molecular physics and chemistry.