Long-range molecular resonances in a cold rydberg gas

We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the [Formula presented] ground state to [Formula presented] Rydberg states ([Formula presented]) reveals resonances at energies corresponding to excited atom pairs [Formula presented]. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., [Formula presented] times the Bohr radius ([Formula presented] for [Formula presented].