Spontaneous Evolution of Rydberg Atoms into an Ultracold Plasma

We have observed the spontaneous evolution of a dense sample of Rydberg atoms into an ultracold plasma, in spite of the fact that each of the atoms may initially be bound by up to $100{\mathrm{cm}}^{-1\mathrm{}}$. When the atoms are initially bound by $70{\mathrm{cm}}^{-1\mathrm{}}$, this evolution occurs when most of the atoms are translationally cold, $<1\mathrm{mK}$, but a small fraction, $\sim 1\%$, is at room temperature. Ionizing collisions between hot and cold Rydberg atoms and blackbody photoionization produce an essentially stationary cloud of cold ions, which traps electrons produced later. The trapped electrons rapidly collisionally ionize the remaining cold Rydberg atoms to form a cold plasma.