Suppression of excitation and spectral broadening induced by interactions in a cold gas of Rydberg atoms.

We report on the observation of ultralong range interactions in a gas of cold rubidium Rydberg atoms. The van der Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii features two important effects: spectral broadening of the resonance lines and suppression of excitation with increasing density. The density dependence of these effects is investigated in detail for the S- and P-Rydberg states with principal quantum numbers n approximately 60 and n approximately 80 excited by narrow-band continuous-wave laser light. The density-dependent suppression of excitation can be interpreted as the onset of an interaction-induced local blockade.