Production, trapping, and detection of ultracold polar KPb molecules

Abstract

We report on the formation of ultracold KRb molecules by photoassociation. The lowest lying singlet and triplet states are detected via state-selective two-photon ionization. Magnetic trapping of triplet molecules has been demonstrated.We present results of experiments involving ultracold heteronuclear KRb molecules 1,2 . Photoassociation of K and Rb atoms in overlapping magneto-optical traps (MOTs), followed by radiative decay, produces molecules in the lowest-lying singlet (X 1 Σ + ) and triplet (a 3 Σ + ) states. Photoassociation spectra of the excited molecular states have been obtained down to 91 cm -1 below the K(4s)+Rb(5p 1/2 ) asymptote. These spectra have allowed the identification of transitions to the eight attractive long-range potentials converging to the K(4s)+Rb(5p 1/2 ) and K(4s)+Rb(5p 3/2 ) asymptotes.The experiment consists of overlapping "dark-spot" MOTs of 39 K and 85 Rb which yield atomic densities estimated at 3x10 10 cm -3 and 10 11 cm -3 , respectively. Overlap of the two atomic clouds is crucial to optimizing the cold molecule formation rate. Photoassociation (PA) is induced by illuminating the atoms with a tunable, single-frequency Ti-sapphire laser. Ultracold molecules which have radiatively decayed to the X 1 Σ + and a 3 Σ + states are detected by two-photon ionization with a Nd:YAG-pumped pulsed dye laser operating at ~600 nm. The resulting KRb + ions are selectively detected with a Channeltron using time-of-flight discrimination.In heteronuclear systems, this ionization scheme is preferred over the trap-loss technique for detecting PA. For homonuclear PA, e.g., with Rb 2 , the excited molecular potentials at large internuclear separation (R) are dominated by the R -3 dipole-dipole interaction. The long-range nature of these potentials leads to a high PA efficiency, and thus significant trap loss. However, ultracold molecule formation also requires decay to a bound molecular state. In the homonuclear case, this is unlikely due to the mismatch of the R -3 excited-state potentials with the R -6 van der Waals ground-state potentials. In heteronuclear PA, however, both the excited-state and ground-state potentials are R -6 , leading to less efficient PA (and therefore less trap loss), but more likely decay to bound states.The ionization detection of X 1 Σ + and a 3 Σ + molecules takes place primarily via the 4 1 Σ + and 4 3 Σ + states, respectively. Both of these upper states are below the K(4s)+Rb(4d) asymptote. With the PA laser fixed on a resonance, we scan the pulsed detection laser to map out the vibrational structure of both the ground state (either X 1 Σ + or a 3 Σ + ) and the upper state. Such state-selective detection spectra yield the distribution of ground-state vibrational levels populated by radiative decay of the photoassociated level. Over the range of PA tunings used, we find population in the v"=86-92 levels of the X 1 Σ + state, which are bound by up to 30 cm -1 , while for the a 3 Σ + state, the v''=20-26 levels, also bound by up to 30 cm -1 , are populated. An example ...