We describe the experimental observation of ultracold NaCs 1 molecular formation in a novel twospecies magneto-optical trap. We interpret our observations in terms of a photoassociative ionization pathway similar to that observed in a sodium trap: Na 1 Cs 1hv Na 1hv Cs ! NaCs 1 1 e 2 . We have directly measured the NaCs 1 production rate as a function of the excitation laser frequency and intensity elucidating the role of ground-state hyperfine levels. We have observed resonance structures in the frequency-dependent ion rate which we associate with the molecular character of the process.[S0031-9007(98)08234-9] PACS numbers: 33.80.Ps Photoassociative (PA) and photoassociative ionization (PAI) spectroscopy of laser cooled and trapped alkali atoms has recently provided [1] remarkably detailed information about long-range interatomic potentials, scattering lengths [2], and excited-state radiative lifetimes. Until recently, collisions of ultracold heteronuclear atomic vapors have been unexplored [3,4].Ultracold heteronuclear PA and PAI is fundamentally different from its homonuclear counterpart for many reasons. One particularly important difference is that the interaction of a ground-excited-state heteronuclear pair lacks the long-range resonant dipole contribution (V ϳ C 3 ͞R 3 ) which plays a decisive role in the homonuclear case [5]. Instead, the long-range part of the heteronuclear pair potential is generally dominated by a comparatively shorter range interaction (V ϳ C 6 ͞R 6 or C 8 ͞R 8 , for example). One consequence of this difference is that the absorption of the photoassociating photon occurs at a much smaller internuclear spacing (R ex ) for a heteronuclear as compared to a homonuclear pair. In PAI of Na in a magneto-optical trap (MOT) R ex ϳ 1800a 0 , whereas for Na 1 Cs we estimate R ex ϳ ͑40 100͒a 0 (see Table I).The particular system under study is a vapor of cesium and sodium atoms contained in a novel twospecies MOT [3]. The trapping laser beams contain two wavelengths l Na 589 nm and l Cs 852 nm (frequency stability ϳ1 MHz). The Na trap laser is detuned by D Na 213 MHz ( 21.3G Na ) from the 3S 1͞2 ͑F 2͒ ! 3P 3͞2 ͑F 0 3͒ Na D2 transition. Before being sent to the trap, the 589 nm light is passed through an electrooptic modulator which generates repumping sidebands shifted from the input laser frequency by 61.71 GHz. The intensity of each sideband is 10% of the total intensity. The 852 nm light is detuned by D Cs 223 MHz (24.4G Cs ) from the 6S 1͞2 ͑F 4͒ ! 6P 3͞2 ͑F 0 5͒ Cs D2 transition. Cs repumping light is provided by a frequency stabilized diode laser tuned to the F 3 ! F 0 4 transition. The magnetic field gradient was 20 G͞cm and the background pressure was ϳ10 210 Torr.To determine the number of trapped atoms, fluorescence from each species was isolated with narrow band (Dl 9 nm) interference filters and measured with independent, calibrated photomultipliers. Trap sizes were measured with two CCD cameras (spatial resolution ø30 mm) oriented so that complete overlap of the Cs and Na clouds could...

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We describe the spectroscopy of highly excited states of ultracold Na 2 molecules using a new photoassociative approach based on resonantly enhanced multiphoton ionization. We excite a remarkable number of Na 2 rotational states providing a rich rovibrational spectrum. We thereby extract not only the first order quadrupole-quadrupole (C 5 ) and the van der Waals constants (C 6 ) but also sensitive atom-atom interaction parameters such as spin-spin, spin-orbit, and perturbative constants.