Spectroscopic electric-field measurement in laser-excited plasmas
  The electric field in mm-sized clouds of cold Rb^+-ions is measured using the Stark effect of Rydberg atoms embedded in the ion clouds. The ion clouds are produced by UV photoionization of laser-cooled rubidium atoms in a magneto-optic trap. In very dense ion clouds, the Rydberg atom excitation spectra reflect the presence of a neutral plasma core and a positively charged ion shell. 
S. K. Dutta, D. Feldbaum, G. Raithel