A pump-probe study of the photoassociation of cold rubidium molecules

The expansion of laser cooling capabilities from the atomic to the molecular regime has proven challenging due to the lack of availability of an equivalent closed-loop cooling cycle within the rich molecular internal energy level structure. An enticing solution is the application of ultrafast coherent control techniques to ultracold molecule generation. Unlike continuous-wave photoassociation techniques, the use of broadband ´pump´ and ´dump´ pulses offers the opportunity to access deeply bound vibrational states. Unlike alternative vibrational cooling strategies such as stimulated Raman adiabatic passage or incoherent optical cycling, this approach preserves coherence and does not require a detailed knowledge of the system spectroscopy. We report on pump-probe experiments investigating the dynamical evolution of bound excited-state Rb₂ dimers after photoassociation. Learning about these dynamics is an important step towards efficient pump-dump formation of deeply bound ground-state molecules. Based on a spectroscopic measurement of the initial state occupied by the interacting atom pairs, we provide evidence that background triplet molecules preassociated by the trapping lasers, additionally to unbound scattering atom pairs, may play an important role in photoassociation dynamics.