Formation of cold Cs₂ ground state molecules through photoassociation in the 1_u pure long-range state

We report on the formation of translationally cold Cs₂ ground state molecules through photoassociation in the 1_u attractive molecular state below the 6s _1/2 + 6p _3/2 dissociation limit. The cold molecules are obtained after spontaneous decay of photoassociated molecules in a MOT and in a dark SPOT. We also used polarized atoms, in the f = 3, m_f = +3 Zeeman ground state. Purely asymptotic and adiabatic calculations including hyperfine interaction and rotation are in excellent agreement with observed structures. As expected, the 1_u state is actually a pure long-range state, consisting of paired atoms, uniquely linked by the first terms of the multipole expansion of the electrostatic interaction. A temperature of 20⁺¹⁵_-5 μK has been measured for the molecular cloud.