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

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Abstract
We have obtained spectra of the high‐lying vibrational levels of the A 1Σ u + and 13Σ g +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 A 1Σ u + vibrational levels v =62–88 for 6Li2 and v=65–97 for 7Li2, and the 13Σ g + 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 a 0 to 220 a 0, where a 0 is the Bohr radius. The spectra arise mainly from s‐wave collisions, and therefore exhibit little rotational structure.