Lifetimes of strontium ultralong-range Rydberg molecules in a dense BEC*

A Rydberg molecule, formed by the binding of one or more ground-state atoms to a Rydberg atom, can be destroyed by interactions involving the Rydberg core ion and a neutral ground-state atom within the Rydberg electron orbit. Previous studies with rubidium and strontium have measured such loss rates in dense gases($\sim10^{14}$cm$^{-3}$) and have attributed the destruction to two main processes: associative ionization resulting in a Rb$_2^+$/Sr$_2^+$ ion formation and dissociation due to L-changing collisions[1,2]. Short lifetimes($\sim1\mu$s) were measured at low principal quantum numbers ($n$) for both Rb and Sr, whereas an increased lifetime($\sim10\mu$s) was reported at high $n$($\sim$140) for Rb. This behavior in Rb was attributed to the presence of a p-wave shape resonance in e-Rb scattering. Here we present a study of the lifetime of Sr Rydberg molecules (Sr does not possess a p-wave shape resonance) in a dense BEC by monitoring the evolution of the Rydberg population using field ionization. The total decay rate and the competition between two main destruction channels is investigated for different $n$, 49$\leq n\leq$150, and BEC densities, $0.8\times10^{14}$cm$^{-3}\leq\rho\leq3\times 10^{14}$cm$^{-3}$. [1] Phys. Rev. A 96, 042702(2017), [2] Phys. Rev. X 6, 031020(2016).