Experiments with Ultracold KRb and Rb$_{2}$ Molecules*

Ultracold molecules are of interest for a number of applications including ultracold chemistry, novel quantum degenerate systems, precision spectroscopy, and quantum computation. Photoassociation (PA) of ultracold atoms is a useful means of producing various diatomic molecular species at sub-mK temperatures. Heteronuclear systems have garnered particular attention because of their permanent electric dipole moments. We use PA to form both KRb and Rb$_{2}$, typically in high vibrational levels of either the singlet ground state ($X \quad ^{1}\Sigma ^{+})$ or lowest-lying triplet state ($a \quad ^{3}\Sigma ^{+})$. In KRb, a novel depletion spectroscopy is used to detect the molecules with both vibrational ($v)$ and rotational ($J)$ resolution. Monitoring the population of a specific $X$-state vibrational level $v''$ with pulsed two-photon ionization, we observe depletion when a cw laser drives a bound-bound transition from ($v''$, $J'')$ to an excited rovibrational level. This high-resolution spectroscopy is helping to guide Raman schemes to transfer ultracold molecules from high-$v''$ levels, produced by PA, to the absolute ground state, which is stable against inelastic collisions. We also use this depletion spectroscopy to precisely measure the ground-state dissociation energy of KRb. In Rb$_{2}$, we observe the effects of resonant coupling between excited 0$_{u}^{+}$ states on ground-state molecule formation. We photoassociate to 0$_{u}^{+}$ levels below the 5$S$ + 5$P_{1/2}$ limit and state-selectively detect the resulting ground-state molecules by two-photon ionization. In the absence of resonant coupling between the two 0$_{u}^{+}$ potentials (converging to the 5$S$ + 5$P_{1/2}$ and 5$S$ + 5$P_{3/2}$ limits), the excited molecules would spontaneously decay overwhelmingly to the highest $v''$ levels, bound by $<$ 1 cm$^{-1}$. The effect of resonant coupling is to provide selected 0$_{u}^{+}$ wavefunctions with increased short-range amplitude, which enhances their decay to more deeply bound levels. Progress towards optical trapping and collisional studies of Rb$_{2}$ will also be reported.