Production and Decay of Ultracold Feshbach Molecules in Bosonic and Fermionic Species.

We investigate the production efficiency of weakly-bound, ultracold molecules in bosonic $^{85}$Rb and also fermionic $^{40}$K when the magnetic field is swept across a Feshbach resonance [1]. For adiabatic sweeps of the magnetic field, our novel model shows that the conversion efficiency of \textit{both} species is solely determined by the phase space density of the atomic cloud, in contrast to a number of theoretical predictions. In the non-adiabatic regime our measurements of the $^{85}$Rb molecule conversion efficiency follow a Landau-Zener model. The spontaneous dissociation of these $^{85}$Rb molecules has also been observed [2]. The molecular lifetime shows a strong dependence on magnetic field, varying by three orders of magnitude between 155.5 G and 162.2 G. Our measurements are in good agreement with theoretical predictions in which molecular dissociation is driven by inelastic spin relaxation [3]. Molecule lifetimes of tens of milliseconds can be achieved close to resonance. [1] Cond-mat/0411487 [2] Phys. Rev. Lett. \textbf{94}, 020401 (2005) [3] Phys. Rev. Lett. \textbf{94}, 020402 (2005)