Ultracold neutral plasmas

By photoionizing a sample of laser-cooled xenon atoms, we create ultracold neutral plasmas with initial temperatures of 1-1000 K and densities as high as 10(10) cm(-3). The plasma is formed by the trapping of electrons by the residual positive charge that is left after some electrons initially leave the sample. We excite plasma oscillations with applied radio frequency fields and use this to monitor the expansion of the unconfined plasma. We have observed significant recombination of the plasma into Rydberg atoms (up to 20 %). At these low temperatures, the only traditional form of recombination that could be significant is three-body recombination (TBR), but we see a number of trends in direct contradiction to what one would expect from classical TBR theory.