Magnetic Trapping and Collisions of Ultracold Rb2 Molecules

We have studied the formation, trapping, and collisions of ultracold molecules in the last few vibrational levels of the lowest triplet electronic state of Rb_2. The molecules are formed by photoassociation of magnetically trapped, laser cooled Rb atoms through the 0_g^-( ∼ 5^2S_1/2 + 5^2P_3/2, v'=1) "pure long range" excited state. This state efficiently populates the v = 32 to v = 37 ground state levels by spontaneous emission. These molecules are detected by three-photon ionization with an 805 nm wavelength pulsed laser. More than 10^4 of these molecules are magnetically trapped, and reach a translational thermal equilibrium state with a temperature of about 200 μK. Their lifetime is limited by inelastic atom-molecule collisions, which occur with a rate constant of about 3 × 10 ^-11 cm^3/s. With the atoms removed from the trap, the lifetime of the molecules is 20 s. Further work is in progress to determine the formation and loss processes of the different molecular ro-vibrational states, and to determine whether it is feasible to Bose condense these molecules through thermal contact with Bose condensed Rb atoms.