Back and forth between Rydberg atoms and ultracold plasmas

By photoionizing cold, trapped atoms it is possible to produce ultracold plasmas with temperatures in the vicinity of 1 K, roughly 4 orders of magnitude colder than conventional cold plasmas. After the first photoelectrons leave, the resulting positive charge traps the remaining electrons in the plasma. Monitoring the dynamics of the expansion of these plasmas shows explicitly the flow of energy from electrons to the ionic motion, which is manifested as the expansion of the plasma. The electron energy can either be their initial energy from photoionization or can come from the energy redistribution inherent in recombination and superelastic scattering from recombined Rydberg atoms. If the cold atoms are excited to Rydberg states instead of being photoionized, the resulting cold Rydberg gas quickly evolves into an ultracold plasma. After a few percent of the atoms are ionized by collisions or blackbody radiation, electrons are trapped by the resulting positive charge, and they quickly lead to ionization of the Rydberg atoms, forming a plasma. While the source of this energy is not clear, a likely candidate is superelastic scattering, also thought to be important for the expansion of deliberately made plasmas. (C) 2003 Optical Society of America.