Formation of translationally cold ${\mathrm{MgH}}^{+}$ and ${\mathrm{MgD}}^{+}$ molecules in an ion trap

We have produced and cooled the molecular ions ${\mathrm{MgH}}^{+}$ and ${\mathrm{MgD}}^{+}$ in a linear Paul trap. These ions were generated by the photochemical reactions ${\mathrm{Mg}}^{+}{(3p}^2{P}_{3/2})+{\mathrm{H}}_2\hspace{0.5em}({\mathrm{D}}_2)\to {\mathrm{MgH}}^{+}\hspace{0.5em}({\mathrm{MgD}}^{+})+\mathrm{H}\hspace{0.5em}(\mathrm{D}),$ 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 ${\mathrm{Mg}}^{+}$ 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.