Photoassociation of long-range $nD$ Rydberg molecules*

A recently discovered class of long-range Rydberg molecules has generated a significant amount of theoretical and experimental interest [1,2]. We present on the observation of long-range homonuclear diatomic $nD$ Rydberg molecules photoassociated out of an ultracold gas of $^{87}$Rb atoms for 34$\le n \le$40 [3]. We measure the ground-state binding energies of $^{87}$Rb$(nD-5S_{1/2})$ molecular states to be larger than those of their $^{87}$Rb$(nS-5S_{1/2})$ counterparts, showing the dependence of the molecular bond on the angular momentum of the Rydberg atom. We probe the transition of $^{87}$Rb$(nD-5S_{1/2})$ molecules from the molecular-binding-dominant regime at low $n$ [Hund's cases (a)] to a fine-structure-dominant regime at high $n$ [Hund's case (c)]. A Fermi model that includes the fine structure of the $nD$ Rydberg atom and hyperfine structure of the $5S_{1/2}$ pertuber is presented that describes this transition. The resulting molecular potentials and bound states are in good agreement with the experimental data.\\[4pt] [1] C. H. Greene, A. S. Dickinson, and H. R. Sadeghpour, PRL, \textbf{85}, 2458-2461 (2000).\\[0pt] [2] V. Bendkowsky et al., Nature, \textbf{458}, 1005-1008 (2009).\\[0pt] [3] D. A. Anderson, S. A. Miller, and G. Raithel, arXiv:1401.2477.