  We present a scheme to embed molecular anions in a gas of ultracold rubidium
atoms as a route towards the preparation of cold molecular ions by collisional
cooling with ultracold atoms. Associative detachment as an important loss
process in collisions between OH- molecules and rubidium atoms is studied. The
density distribution of trapped negative ions in the multipole radiofrequency
trap is measured by photodetachment tomography, which allows us to derive
absolute rate coefficients for the process. We define a regime where
translational and internal cooling of molecular ions embedded into the
ultracold atomic cloud can be achieved.
