Many-Body Ionization in a Frozen Rydberg Gas

Here we present measurements of spontaneous ionization in a dense gas of 300 $\mu $K $^{85}$Rb atoms of \textit{n$\sim $} 50. At densities of $\sim $10$^{10}$ cm$^{-3}$ ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 $\mu $K blackbody radiation. Rapid ionization is accompanied by spectral broadening and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.