Resonant Coupling in the Formation of Ultracold Ground State Molecules via Photoassociation

We demonstrate the existence of a new mechanism for the formation of ultracold molecules via photoassociation of cold cesium atoms. The experimental results, interpreted with numerical calculations, suggest that a resonant coupling between vibrational levels of the $0_u^{+}$ $\left({6s+6p}_{1/2}\right)$ and $\left({6s+6p}_{3/2}\right)$ states enables formation of ultracold molecules in vibrational levels of the ground state well below the $6s+6s$ dissociation limit. Such a scheme should be observable with many other electronic states and atomic species.