Observation of Blue-Detuned Photoassociation to the 2 $(0_g^+)$ State of $^{85}$Rb$_2$ via REMPI*

We report detection of photoassociation to vibrational levels blue of the $^{85}$Rb$_2$ $5s+ 5p_{1/2}$ asymptote, in the previously-unobserved 2 $(0_g^+)$ Hund's case (c) state that corresponds to 2 $^1 \Sigma _g^+$ in Hund's case (a). These excited-state ultracold molecules decay to the $a \, ^3 \Sigma _u^+$ state and are detected by pulsed REMPI through the 2 $^3 \Sigma _g^+$ state. We also observe, via trap loss, the 2 $(0_u^+)$, 2 $(0_g^-)$, and 2 $(1_g)$ states observed in [1], and confirm that these states are not the source of the observed molecules. Photoassociation through the observed levels of the 2 $(0_g^+)$ state populates vibrational levels approximately halfway up the $a \, ^3 \Sigma _u^+$ potential well. This pathway complements the blue-detuned photoassociation technique described in [2], which accesses the bottom of the $a$ state potential.\\[4pt] [1] R. A. Cline, J. D. Miller, and D. J. Heinzen, Phys. Rev. Lett. \textbf{73}, 632 (1994)\\[0pt] [2] M. A. Bellos, \emph{et. al.}, Phys. Chem. Chem. Phys. \textbf{13}, 18880 (2011)