Recombination during expansion of ultracold plasma

Signals of ultracold plasma are observed by two-photon ionization of laser-cooled caesium atoms in a magnetooptical trap.Recombination of ions and electrons into Rydberg atoms during the expansion of ultracold plasma is investigated by using state-selective field ionization spectroscopy.The dependences of recombination on initial electron temperature(1-70 K) and initial ion density(～10 10 cm-3) are investigated.The measured dependence on initial ion density is N 1.547±0.004 at a delay time of 5 μs.The recombination rate rapidly declines as initial electron temperature increases when delay time is increased.The distributions of Rydberg atoms on different values of principal quantum number n,i.e.n = 30-60,at an initial electron temperature of 3.3 K are also investigated.The main experimental results are approximately explained by the three-body recombination theory.