We present results from two-photon photoassociative spectroscopy of the least-bound vibrational level of the

Ultracold molecules offer a broad variety of applications, ranging from metrology to quantum computing. However, forming "real" ultracold molecules, i.e., in deeply bound levels, is a very difficult proposition. Here, we show how photoassociation in the vicinity of a Feshbach resonance enhances molecular formation rates by several orders of magnitude. We illustrate this effect in heteronuclear systems, and find giant rate coefficients even in deeply bound levels. We also give a simple analytical expression for the photoassociation rate and discuss future applications of the Feshbach-optimized photoassociation technique.

We report the use of photoassociative spectroscopy to determine the ground-state s-wave scattering lengths for the main bosonic isotopes of strontium, 86 Sr and 88 Sr. Photoassociative transitions are driven with a laser red detuned by up to 1400 GHz from the 1 S 0 -1 P 1 atomic resonance at 461 nm. A minimum in the transition amplitude for 86 Sr at ÿ494 5 GHz allows us to determine the scattering lengths 610a 0 < a 86 < 2300a 0 for 86 Sr and a much smaller value of ÿ1a 0 < a 88 < 13a 0 for 88 Sr. DOI: 10.1103/PhysRevLett.95.223002 PACS numbers: 32.80.Pj Photoassociative spectroscopy (PAS) of ultracold gases, in which a laser field resonantly excites colliding atoms to rovibrational states of excited molecular potentials, is a powerful probe of atomic cold collisions [1]. Transition frequencies have been used to obtain dispersion coefficients of molecular potentials, which yield the most accurate value of the atomic excited-state lifetime [2 -5]. Transition amplitudes are related to the wave function for colliding ground-state atoms [6,7] and can be used to determine the ground-state s-wave scattering length [8][9][10][11][12][13].The s-wave scattering length is a crucial parameter for determining the efficiency of evaporative cooling and the stability of a Bose-Einstein condensate (BEC). It also sets the scale for collisional frequency shifts, which can limit the accuracy and stability of atomic frequency standards.The cold collision properties of alkaline-earth atoms such as strontium, calcium, and magnesium, and atoms with similar electronic structure, such as ytterbium, are currently the focus of intense study. These atoms possess narrow optical resonances that have great potential for optical frequency standards [14 -18]. Laser cooling on narrow transitions [19] is an efficient route to high phasespace density [20,21], and a BEC was recently produced with ytterbium [22]. Fundamental interest in alkaline-earth atoms is also high because their simple molecular potentials allow accurate tests of cold collision theory [23][24][25].PAS of calcium [12] and ytterbium [13] was recently used to determine s-wave scattering lengths of these atoms. This Letter reports the use of PAS to determine the groundstate s-wave scattering lengths for the main bosonic isotopes of strontium, 86 Sr and 88 Sr, which have relative abundances of 10% and 83%, respectively. We find a huge scattering length for 86 Sr of 610a 0 < a 86 < 2300a 0 . Appreciable uncertainty comes from the value of C 6 for the ground-state potential. In contrast, for 88 Sr we find ÿ1a 0 < a 88 < 13a 0 . Recently posted [5] PAS results for 88 Sr yielded an improved measurement of the 5s5p 1 P 1 atomic lifetime ( 5:263 0:004 ns), which we use in our analysis. Disagreement between our reported value of a 88 and results of Ref.[5] will be discussed below.For PAS of strontium, atoms are initially trapped in a magneto-optical trap (MOT) operating on the 461 nm 1 S 0 -1 P 1 transition, as described in Refs. [4,26]. We are able to produce pure samples of each iso...

We report photoassociative spectroscopy of 88 Sr2 in a magneto-optical trap operating on the 1 S0 → 3 P1 intercombination line at 689 nm. Photoassociative transitions are driven with a laser red-detuned by 600-2400 MHz from the 1 S0 → 1 P1 atomic resonance at 461 nm. Photoassociation takes place at extremely large internuclear separation, and the photoassociative spectrum is strongly affected by relativistic retardation. A fit of the transition frequencies determines the 1 P1 atomic lifetime (τ = 5.22 ± 0.03 ns) and resolves a discrepancy between experiment and recent theoretical calculations.

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