We study the states of ${\mathrm{Rb}}_2$ that lie within 35 ${\mathrm{cm}}^{-1\mathrm{}}$ of the $5^2{S}_{\frac{1}{2}}+5^2{P}_{\frac{3}{2}}$ dissociation limit with photoassociation spectroscopy of laser-cooled Rb atoms. We observe the rotationally resolved bound levels of the novel $0_g^{-}$ "pure long-range" state, which has an inner turning point beyond 25 bohrs. We also identify levels belonging to $1_g$ and $0_{\mu }^{+}$ states and find that the $0_{\mu }^{+}$ levels are broadened by predissociation. With our new method, excited molecular states with a binding energy from less than 0.3 to more than 1000 ${\mathrm{cm}}^{-1\mathrm{}}$ can be studied with a resolution better than 0.002 ${\mathrm{cm}}^{-1\mathrm{}}$.

• Received 21 December 1993

DOI:https://doi.org/10.1103/PhysRevLett.73.632