Observation of ultralong-range Rydberg molecules

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. A Rydberg atom has one electron excited into an orbital with a very high principal quantum number. The scattering of such an electron from a second atom in the ground state gives rise to long-range bonding, yielding giant molecules with internuclear separations reaching several thousand Bohr radii. Using s-state rubidium Rydberg atoms with quantum numbers between 34 and 40, Bendkowsky and colleagues have now spectroscopically characterized such 'Rydberg molecules', and measured their lifetimes and polarizabilities. Rydberg atoms have an electron in a state with a very high principal quantum number, and as a result can exhibit unusually long-range interactions. One example is the bonding of two such atoms by multipole forces to form Rydberg–Rydberg molecules with very large internuclear distances1,2,3. Notably, bonding interactions can also arise from the low-energy scattering of a Rydberg electron with negative scattering length from a ground-state atom4,5. In this case, the scattering-induced attractive interaction binds the ground-state atom to the Rydberg atom at a well-localized position within the Rydberg electron wavefunction and thereby yields giant molecules that can have internuclear separations of several thousand Bohr radii6,7,8. Here we report the spectroscopic characterization of such exotic molecular states formed by rubidium Rydberg atoms that are in the spherically symmetric s state and have principal quantum numbers, n, between 34 and 40. We find that the spectra of the vibrational ground state and of the first excited state of the Rydberg molecule, the rubidium dimer Rb(5s)–Rb(ns), agree well with simple model predictions. The data allow us to extract the s-wave scattering length for scattering between the Rydberg electron and the ground-state atom, Rb(5s), in the low-energy regime (kinetic energy, <100 meV), and to determine the lifetimes and the polarizabilities of the Rydberg molecules. Given our successful characterization of s-wave bound Rydberg states, we anticipate that p-wave bound states9, trimer states10 and bound states involving a Rydberg electron with large angular momentum—so-called trilobite molecules5—will also be realized and directly probed in the near future.