Millimeter-wave rubidium Rydberg van der Waals spectroscopy

We have observed the diatomic nsns→ns(n+1)s→(n+1)s(n+1)s microwave transitions of pairs of cold Rb Rydberg atoms for $36<n<48$. The $ns(n+1)s$ state has by far the largest van der Waals shift due to its near degeneracy with the nearby np3/2np3/2 state. In fact, the energy difference and the van der Waals shift change sign at $n=38$ and we observe shifts of both signs. The combined excitation of the $ns(n+1)s$ and $(n+1)s(n+1)s$ states leads to an unusual line shape which can be described by a three-level Floquet model.