Photoassociative spectroscopy of ultracold K-39 atoms in a high-density vapor-cell magneto-optical trap

We have performed photoassociative spectroscopy of ultracold potassium atoms in a high-density vapor-cell magneto-optical trap (MOT) in which atoms are confined predominantly in ''dark'' hyperfine levels that do not interact with the trapping light. Three well-resolved vibrational series (one strong and two weak) of the K-39(2) molecule are observed within 8 cm(-1) below the 4 (2)p(3/2) atomic Limit. One of the series is identified as the 0(g)(-) pure long-range state and the other two are assigned to the 0(u)(+) and 1(g) states. These photoassociative spectra are obtained by measuring reductions in the trap fluorescence due to trap loss induced by an intense photoassociation laser. By using a vapor-cell-dark MOT technique, we achieve atomic densities greater than 10(11) cm(-3), increasing our trap loss signals by an order of magnitude compared to a normal MOT.