The little molecule that could

Abstract

The creation of diatomic molecules bound by roaming electrons that allow a huge internuclear distance is some achievement. It opens the door to further experimental exploitation of the principles involved. In a Rydberg atom, at least one electron is excited into an orbital with a very high principal quantum number that extends the atom's electronic envelope far beyond the nucleus. Based on ideas introduced by Enrico Fermi in 1934, a recent piece of theoretical work predicted that the scattering of such an electron from a second atom in the ground-state could give rise to attractive interactions. This would yield giant molecules with internuclear separations reaching several thousand Bohr radii. The spectroscopic characterization of such ultra-long-range 'Rydberg molecules' is now reported. The molecules, ultracold rubidium dimers, have spectra in good agreement with model predictions. This achievement raises the exciting prospect of realizing other exotic molecular species such as the so-called trilobite molecules in the near future.