Photoassociation Rate of a ^7Li Bose-Einstein Condensate near a Feshbach Resonance

In a photoassociation (PA) process a pair of atoms collide in the presence of a resonant light field creating an excited molecule. For ultracold atoms, the rate of PA depends on the s-wave scattering length, a_s, which determines the wavefunction overlap between the collisional ground state and the excited molecular bound state. We investigate this dependence experimentally using a pure Bose condensate of ^7Li in the F=1, m_F=1 hyperfine state confined in an optical dipole trap. We vary a_s below the 730 G Feshbach resonance and measure the rate of loss from the condensate due to a PA pulse which couples atoms to the v' = 83 vibrational level of the 1^3σ_g^+ molecular state. The measured loss rate varies by more than a factor of 30 over the magnetic field range of 660 - 730 G. At 710 G the rate approaches zero, which we attribute to a node in the ground state wavefunction. We also compare the PA loss rate in a BEC to a thermal gas over this same range of magnetic fields.