Magnetic-field imaging of weakly bound levels of the ground-state Na2 dimer

We measured in a molecular-beam experiment the magnetic-field dependence of the last bound levels of the X 1Σg+ and a3Σu+ potentials close to the 3s+3s asymptote of Na2. The field behavior of these levels in an external magnetic field differs for even or odd nuclear mechanical angular momentum. Coupled-channel calculations using high-precision potentials for the X 1Σg+ and a3Σu+ potentials give excellent quantitative agreement between experiment and theory. Our data lead to a precise and quantitative description of the magnetic decoupling of the singlet and triplet manifold and of Feshbach resonances in a cold atomic collision