Hyperfine structure of high lying vibrational levels in the Na2 1g (3S+3P_3/2) state observed by photoassociation of cold atoms

The spectroscopic technique of photoassociating ultracold (< 1mK) atoms to produce diatomic molecules has permitted the detailed investigation of long range molecular potentials. At large internuclear separations the constituent atoms retain their atomic identity and the molecular properties can be directly related to atomic properties. We have investigated the hyperfine structure of the Na₂ 1_g (3S+3P_3/2) state for levels in the range v=50-70 with a measurement precision of ≈ 3 MHz in relative line positions. By ionizing these molecules through a recently discovered, doubly-excited, autoionizing 0_u^- state dissociating to (P_3/2+P_3/2) we can experimentally label lines as having odd or even total nuclear spin. The positions are well reproduced by a model which incorporates the competition between the atomic hyperfine interaction and the molecular rotation. The detailed line shapes of individual lines can be related to the hyperfine states of the initial, free atom collisions. A breakdown of the Wigner threshold law for p-wave collisions is observed.