Recombination during expansion of ultracold plasma

Signals of ultracold plasma are observed by two-photonionization of laser-cooled caesium atoms in a magneto-optical trap.Recombination of ions and electrons into Rydberg atoms during theexpansion of ultracold plasma is investigated by usingstate-selective field ionization spectroscopy. The dependences ofrecombination on initial electron temperature (1--70~K) and initialion density ($ sim $10$^{10}$~cm$^{ - 3})$ are investigated. Themeasured dependence on initial ion density is $N^{1.547pm 0.004}$at a delay time of 5~$mu $s. The recombination rate rapidlydeclines as initial electron temperature increases when delay timeis increased. The distributions of Rydberg atoms on different valuesof principal quantum number $n$, i.e. $n=30$--60, at an initialelectron temperature of 3.3~K are also investigated. The mainexperimental results are approximately explained by the three-bodyrecombination theory.