Near-threshold photoassociation of ${}^{87}{\mathrm{Rb}}_2$

Photoassociation of cold ${}^{87}\mathrm{Rb}{(5s,f}_b=2)$ atom pairs is studied within 13 GHz of the dissociation limits ${}^{87}\mathrm{Rb}{(5p}_{3/2}{,f}_a{)+}^{87}\mathrm{Rb}{(5s,f}_b=2).\mathrm{}$ Vibrational levels in the long range, hyperfine manifolds of the $0_g^{-}$ and $1_g$ states arising from the ${}^{87}\mathrm{Rb}{(5p}_{3/2})$ fine structure limit are identified to lead to the formation of stable molecules in the photoassociation process. The vibrational bands are observed to break off, at positions matching the energetic location of potential-energy barriers predicted in the hyperfine manifolds at about 1 GHz above the ${}^{87}\mathrm{Rb}{(5p}_{3/2}{,f}_a{=3)+}^{87}\mathrm{Rb}{(5s,f}_b=1)$ dissociation limit. Empiricial potential-energy curves are derived from the data. These are found to be in agreement with predictions based on the long-range hyperfine recoupling of the excited molecular states. The selectivity of the excitation process and predissociation mechanisms are discussed.