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 np(3/2)np(3/2) 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.