Observation of a Shape Resonance in Cold-Atom Scattering by Pulsed Photoassociation

Abstract

We observe the time dependence of a cold-atom collision in a pulsed photoassociation experiment. For a $g$-wave shape resonance in the ${}^{85}\mathrm{Rb}{+}^{85}\mathrm{Rb}$ system we measure the time needed to build up the resonant state by tunneling through the centrifugal barrier. Combining this with time-independent ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ photoassociation we determine the resonance energy and find evidence for the decay of the shape resonance into inelastic channels. We also determine the ${}^{85}\mathrm{Rb}+{}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}{+}^{87}\mathrm{Rb}$ $C_6$ coefficient and triplet scattering lengths without relying on ab initio calculations.