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.

Plasmas of ${\mathrm{Mg}}^{+}$ ions, containing more than ${10}^5$ ions, have been observed to reach well-ordered (crystalline) states by applying laser cooling. The crystals are highly elongated with up to ten concentric cylindrical shells surrounding a central string. Such large structures have not previously been observed in a Paul trap. The amplitude of the micromotion of the ions can be larger than the shell spacings. As the diameter changes along the crystals, sharp transitions are observed when new shells form, in good agreement with molecular dynamics simulations. The predictions from simulations of how ordering develops with decreasing temperature are also confirmed.

We demonstrate a simple and nondestructive method for identification of a single molecular ion sympathetically cooled by a single laser cooled atomic ion in a linear Paul trap. The technique is based on a precise nondestructive determination of the molecular ion mass through a measurement of the eigenfrequency of a common motional mode of the two ions. The demonstrated mass resolution is sufficiently high that molecular ion mass doublets can potentially be distinguished from each other. The obtained results represent an important step towards single molecule gas phase chemical physics.

Highly charged ions in cold confines: High-energy irradiation can strip many electrons away from individual atoms, producing ions with charges of +10 or more. However, many of the interesting properties of such highly charged ions are hard to study or exploit under the extreme conditions needed to prepare them. Schmöger et al. cooled down argon ions with +13 charges from the megakelvin temperatures needed for their generation to millikelvin temperatures appropriate for high-precision spectroscopy. The method relies on sympathetic cooling by a cold sample of singly charged beryllium ions and is likely to be applicable to a broad range of other elements. Science , this issue p. 1233