Observation of Cold Collisions between Trapped Ions and Trapped Atoms

We demonstrate a double-trap system well suited to study cold collisions between trapped ions and trapped atoms. Using Yb${}^{+}$ ions confined in a Paul trap and Yb atoms in a magneto-optical trap, we investigate charge-exchange collisions of several isotopes for collision energies down to 400 neV (5 mK). The measured rate coefficient of $6\times {10}^{-10}$ cm${}^3$s${}^{-1}$, constant over four orders of magnitude in collision energy, is in good agreement with that derived from a semiclassical Langevin model for an atomic polarizability of 143 a.u.