Singlet-Triplet Mixing and Ground-State Molecule Formation in Ultracold Rb$_{2}$

We have observed two manifestations of singlet-triplet mixing in ultracold Rb$_{2}$. In the first, we start with high-$v$ levels of the $X{ }^1\Sigma _g^+ $ state which are produced by photoassociation and detected by REMPI. Scanning the detection laser reveals transitions to the 2${ }^3\Pi _u $ state. In the second case, we start with $a{ }^3\Sigma _u^+ $ molecules and observe transitions to 3${ }^1\Sigma _g^+ $. We propose a scheme for producing $X{ }^1\Sigma _g^+ (v$=0), starting with $a{ }^3\Sigma _u^+ $ molecules in high-$v$: $a{ }^3\Sigma _u^+ \to 3{ }^1\Sigma _g^+ \to A{ }^1\Sigma _u^+ \to X{ }^1\Sigma _g^+ $. The first step is made possible by the admixture of triplet character in the 3${ }^1\Sigma _g^+ $ state.