Observation of a molecular bond between ions and Rydberg atoms

Atoms with a highly excited electron, called Rydberg atoms, can form unusual types of molecular bonds^1-4. The bonds differ from the well-known ionic and covalent bonds^5,6 not only by their binding mechanisms, but also by their bond lengths ranging up to several micrometres. Here we observe a new type of molecular ion based on the interaction between the ionic charge and a flipping-induced dipole of a Rydberg atom with a bond length of several micrometres. We measure the vibrational spectrum and spatially resolve the bond length and the angular alignment of the molecule using a high-resolution ion microscope⁷. As a consequence of the large bond length, the molecular dynamics is extremely slow. These results pave the way for future studies of spatio-temporal effects in molecular dynamics (for example, beyond Born-Oppenheimer physics).