  We demonstrate rotational and vibrational cooling of cesium dimers by optical
pumping techniques. We use two laser sources exciting all the populated
rovibrational states, except a target state that thus behaves like a dark state
where molecules pile up thanks to absorption-spontaneous emission cycles. We
are able to accumulate photoassociated cold Cs2 molecules in their absolute
ground state (v = 0, J = 0) with up to 40% efficiency. Given its simplicity,
the method could be extended to other molecules and molecular beams. It also
opens up general perspectives in laser cooling the external degrees of freedom
of molecules.
