Optical pumping and vibrational cooling of molecules

The methods producing cold molecules from cold atoms tend to leave molecular ensembles with substantial residual internal energy. For instance, Cs2 molecules initially formed via photoassociation of cold Cs atoms are in several vibrational levels, v, of the electronic ground state. Here we apply a broadband femtosecond laser that redistributes the vibrational population in the ground state via a few electronic excitation - spontaneous emission cycles. The laser pulses are shaped to remove the excitation frequency band of the v = 0 level, preventing re-excitation from that state. We observe a fast and efficient accumulation, about 70% of the initially detected molecules, in the lowest vibrational level, v = 0, of the singlet electronic state. The validity of this incoherent depopulation pumping method is very general and opens exciting prospects for laser cooling and manipulation of molecules.