Formation of Rydberg Atoms in an Expanding Ultracold Neutral Plasma

We study the formation of Rydberg atoms in expanding plasmas at temperatures of 1–1000 K and densities from ${10}^5–{10}^{10}{\mathrm{cm}}^{-3\mathrm{}}$. Up to $20\%$ of the initially free charges recombine in about $100\mu \mathrm{s}$, and the binding energy of the Rydberg atoms approximately equals the increase in the kinetic energy of the remaining free electrons. Three-body recombination is expected to dominate in this regime, yet most of our results are inconsistent with this mechanism.