Photoassociation spectrum of ultracold Rb atoms

We study the photoassociative collisional loss of laser-cooled Rb atoms from a far-off resonance optical dipole force atom trap. We obtain a well-resolved photoassociation spectrum from 50 ${\mathrm{cm}}^{\mathrm{-}1}$ to 980 ${\mathrm{cm}}^{\mathrm{-}1}$ below the first excited dissociation limit of the ${\mathrm{Rb}}_2$ molecule. Two vibrational series associated with excited ${\mathrm{Rb}}_2$ ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{\mathit{g}}^{+}{}_{}{}^{}$ states are clearly visible. Oscillations in the associated Franck-Condon factors reflect the structure of the triplet ground state wave function. Our results clearly demonstrate the potential of photoassociation spectroscopy as a new probe of molecular structure.