Fine-Structure Predissociation of Ultracold PhotoassociatedK239Molecules Observed by Fragmentation Spectroscopy

We report the first observation of fragmentation spectra of the fine-structure predissociated long-range $1_g$ and $0_u^{+}$ $\left(b{}_{}{}^3{}_{}{}^{}\Pi _{0^{+}u}^{}{}_{}{}^{}\right)$ states of ultracold $(T<\mathrm{}1\mathrm{}\mathrm{mK})$ ${}^{39}{\mathrm{K}}_2$ molecules. The $1_g$ state predissociates via the curve crossing at long range $\left(R\sim 23Å\right)$ with the repulsive $0_g^{+}$ state. The $0_u^{+}$ state, however, predissociates predominantly through molecular fine-structure mixing at short range with the $b{}_{}{}^3{}_{}{}^{}\Pi _{1u}^{}{}_{}{}^{}$ component. Our technique of fragmentation spectroscopy is a sensitive and selective tool for ultracold photoassociation and fine-structure-changing collisions.