We have developed a two-step " R-transfer" method that efficiently produces translationally ultracold potassium molecules in the X (1)Sigma(+)(g) electronic ground state. Laser-cooled atoms are initially photoassociated at large internuclear separation R to form molecules in high vibrational levels of the 1 (1)Pi(g) state, which are in turn excited by an additional laser to shorter-range Rydberg states such as 5 (1)Pi(u) and 6 (1)Pi(u). Subsequent radiative decay produces ground-state molecules at rates up to 10(5) molecules/second per vibrational level.
We use double-resonance photoassociative spectroscopy of ultracold 39 K atoms to precisely determine the triplet a 3 ⌺ u ϩ scattering length for the various isotopes of potassium. Photoassociation of free 39 K atoms to the pure long-range 0 g Ϫ (vЈϭ0, JЈϭ2) level is followed by stimulated emission to high-lying levels of the a 3 ⌺ u ϩ potential. The binding energies of levels within 5 GHz below the lowest ground-state 4s 1/2 ( f a ϭ1) ϩ4s 1/2 ( f b ϭ1) hyperfine asymptote are measured by both trap loss and ionization detection. The locations of these near-threshold hyperfine-coupled molecular levels allow us to constrain the triplet potential and thereby determine the triplet scattering length. The result for 39 K, a t ϭϪ33Ϯ5a 0 (1a 0 ϭ0.052 917 7 nm), is a factor of ϳ5 improvement over previous determinations and establishes that a large 39 K Bose-Einstein condensate will not be stable.
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