We observe small numbers of laser-cooled ${\mathrm{Hg}}^{+}$ 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 $k_{\mathrm{B}}T$ in these clusters is observed to be as high as 120.

• Received 29 September 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.2935