Coulomb Expansion of Laser-Excited Ion Plasmas

We determine the electric field in mm-sized clouds of cold $\mathrm{R}{\mathrm{b}}^{\mathrm{+}}$ ions, produced by photoionization of laser-cooled ${}^{\mathrm{87}}\mathrm{R}\mathrm{b}$ atoms in a magneto-optical trap, using the Stark effect of embedded Rydberg atoms. The dependence of the electric field on the time delay between the ion plasma production and the probe of the electric field reflects the Coulomb expansion of the plasma. Our experiments and models show expansion times $<1\mu \mathrm{s}$.