Molecular vibrational cooling by optical pumping with shaped femtosecond pulses

Some of us have recently reported (Viteau et al 2008 Science 321 232-4) vibrational cooling of translationally cold Cs₂ molecules into the lowest vibrational level v = 0 of the singlet X¹ Σ^g ground electronic state. Starting from a sample of cold molecules produced in a collection of vibrational levels of the ground state, our method was based on repeated optical pumping by laser light with a spectrum broad enough to excite all populated vibrational levels but frequency-limited in such a way to eliminate transitions from v = 0 level, in which molecules accumulate. In this paper, this method is generalized to accumulate molecules into an arbitrary selected 'target' vibrational level. It is implemented by using ultrashort pulse shaping techniques based on liquid crystal spatial light modulator. In particular, a large fraction of the initially present molecules is transferred into a selected vibrational level such as v = 1, 2 and 7. Limitations of the method as well as the possible extension to rotational cooling are also discussed.