Atomic-Ion Coulomb Clusters in an Ion Trap

We observe small numbers of laser-cooled Hg⁺ ions, which are confined in a Paul radio-frequency ion trap, to crystallize into regular arrays or clusters. We investigate the structure of these clusters by direct imaging, optical spectroscopy, and numerical calculations. The spectroscopy of such "pseudomolecules" is unique in that we can probe individual atoms of the molecule separately. The ratio of Coulomb potential energy per ion to kBT in these clusters is observed to be as high as 120.