An ultracold neutral plasma

We present the first experiment to observe an ultracold neutral plasma. The plasma, which was created by photoionization of laser cooled atoms, has charge densities as high as 2x10{sup 9} cm{sup -3}, and the temperatures of electrons and ions are as low as 100 mK and 10 {mu}K, respectively. The plasma has a lifetime of about 100 {mu}s, much longer than predicted by recombination rates. When the laser that excites the atoms is tuned below the ionization limit we create a sample of very highly excited cold Rydberg atoms. At our highest densities and during a time of a few microseconds, in which the Rydberg atoms are essentially stationary, the ensemble evolves towards an unbound plasma-like state.