Long-range interactions in molecules and rotational coupling

We report a case study on Na₂ of the influence of rotational coupling for molecular states directly below the dissociation limit, where the electronic binding energy and the hyperfine interactions are of similar magnitude and the rotational energy can be varied from small to large compared to the former energies. The experimental observation and the theoretical analysis are important for obtaining precise data concerning long-range interactions and extrapolation to the dissociation limit, which are required for describing cold collisions in atomic traps. A consistent model for all observations with rotational quantum number J′ up to 41 is developed which involves few atomic parameters and demonstrates that these are sufficient to describe molecular levels few μeV below the dissociation energy.