Observation of Ultracold Ground-State Heteronuclear Molecules

We report the observation of translationally ultracold heteronuclear ground-state molecules in a two-species magneto-optical trap containing ${}^{\mathrm{39}}\mathrm{K}$ and ${}^{85}\mathrm{Rb}$ atoms. The KRb molecules are produced via photoassociation and detected by multiphoton ionization. We had characterized their temperature and measured their formation rate constant. We believe that the two-species trap could be used as a reliable source of ultracold molecules to be captured by electrostatic, magnetic, or optical traps. This possibility will certainly motivate further investigation of quantum collective effects as well as high-resolution spectroscopy of the rovibrational level structure of cold heteronuclear molecular systems.