State-selective detection of ultracold KRb molecules in near-dissociation vibrational levels

Using resonance-enhanced one-color two-photon ionization, we have detected ultracold polar KRb molecules with vibrational selectivity. The KRb molecules are produced by photoassociation (PA) followed by radiative decay to either $X\,\,^1\Sigma^+$ ground state or the metastable triplet $a\,\,^3\Sigma^+$ state[1]. The distribution of final states can be modified by selecting different PA resonances, which radiate to various near-dissociation vibrational levels in the $X$ and $a$ states, following the Franck-Condon principle. We have obtained long range bound-bound excitation spectra of these molecules, which also provide the first spectroscopic data on the $(4)\,\,^1\Sigma^+$ shelf state, the $(4)\,\,^3\Sigma^+$ state and the $(3)\,\,^3\Pi$ state. Analysis of our spectra shows vibrational spacings in these excited states in good agreement with those calculated from $ab$ $initio$ potential curves. We acknowledge support from NSF.\newline [1] D. Wang et al., Phys. Rev. Lett., 93, 243005(2004).