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.

Figure 1

(a) Experiment time sequence (not in scale) used to produce and detect the KRb molecules. (b) Time-of-flight spectrum showing the formation of heteronuclear molecules. The ratio between the TOF peaks due to ${\mathrm{KRb}}^{+}$, ${\mathrm{Rb}}_2^{+}$, and ${\mathrm{K}}_2^{+}$ observed agrees with those based on the mass ratios (time shifts of $\pm 6\%$).

Figure 2

Molecular formation channel proposed for the KRb ultracold molecules and the detection channel.

Figure 3

(a) Ion signal as a function of the pulsed dye laser energy. The quadratic behavior with the pulsed laser energy (dotted line fit) assures the molecular ionization by two-photon transitions. (b) TOF peak value of the $\mathrm{K}{\mathrm{Rb}}^{+}$ molecular ion yield as a function of the Rb MOT density, for two different K MOT densities: $1.7\times {10}^{10}{\mathrm{cm}}^{-3}$ (full up triangles), and $1.2\times {10}^{10}{\mathrm{cm}}^{-3}$ (hollow up triangles). The lines are fits assuming the linear dependence on the Rb MOT density, in good agreement with the experimental data.