Photoassociative spectroscopy of ultracoldK39atoms 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 ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}_2^{}{}_{}{}^{}$ molecule are observed within 8 ${\mathrm{cm}}^{-1\mathrm{}}$ below the $4{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ 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 ${10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$, increasing our trap loss signals by an order of magnitude compared to a normal MOT.