Photoassociation of Long-Range nD Rydberg Molecules

We observe long-range homonuclear diatomic nD Rydberg molecules photoassociated out of an ultracold gas of Rb-87 atoms for 34 <= n <= 40. The measured ground-state binding energies of Rb-87(nD + 5S(1/2)) molecular states are larger than those of their Rb-87(nS + 5S(1/2)) counterparts, which shows the dependence of the molecular bond on the angular momentum of the Rydberg atom. We exhibit the transition of Rb-87(nD + 5S(1/2)) molecules from a molecular-binding-dominant regime at low n to a fine-structure-dominant regime at high n [akin to Hund's cases (a) and (c), respectively]. In the analysis, the fine structure of the nD Rydberg atom and the hyperfine structure of the 5S(1/2) atom are included.