  Long-range metastable molecules consisting of two cesium atoms in high
Rydberg states have been observed in an ultracold gas. A sequential
three-photon two-color photoassociation scheme was employed to form these
molecules in states which correlate to $np(n+1)s$ dissociation asymptotes.
Spectral signatures of bound molecular states are clearly resolved at the
positions of avoided crossings between long-range van der Waals potential
curves. The experimental results are in agreement with simulations based on a
detailed model of the long-range multipole-multipole interactions of
Rydberg-atom pair states. We show that a full model is required to accurately
predict the occurrence of bound Rydberg macrodimers. The macrodimers are
distinguished from repulsive molecular states by their behavior with respect to
spontaneous ionization and possible decay channels are discussed.
