Photoassociative spectroscopy of long-range states of ultracold 6Li2 and 7Li2

We have obtained spectra of the high-lying vibrational levels of the A1Σu+ and 13Σg+states of both 6Li2 and 7Li2 via photoassociation of colliding ultracold atoms confined to a magneto-optical trap. Because of the low collision energies (∼1 mK), very high spectroscopic resolution is realized, and sensitivity is greatest for the usually difficult to access long-range states. Binding energies relative to the center of gravity of the resolved hyperfine structure are given for the A1Σu+ vibrational levels v =62–88 for 6Li2 and v=65–97 for 7Li2, and the 13Σg+ vibrational levels v=56–84 for 6Li2 and v=62–90 for 7Li2. These are extremely long-range levels with outer classical turning points ranging from 25 a0 to 220 a0, where a0 is the Bohr radius. The spectra arise mainly from s-wave collisions, and therefore exhibit little rotational structure.