Recombination in an expanding ultracold plasma

Summary form only given. We have observed recombination into a large range of Rydberg levels in an ultracold plasma. The plasma is created by two-photon ionization of laser-trapped and -cooled metastable xenon atoms. By changing the intensity and detuning of the ionizing laser pulse, the initial temperature and density of the plasma can be varied. This allows us to access plasma coupling parameters (???) between 0.1 and 5. In this range of values, we previously reported unexplained heating of the expanding cloud, between 10 and 100 ???s after creation of the plasma. The recently observed recombination appears to provide much of the extra energy responsible for this behavior. The recombined atoms, up to 20% of the created plasma, populate energy levels down to n=40. They survive for ∼1 ms indicating that high angular momentum states are present. However, standard scaling laws for three-body recombination do not agree with the observed dynamics. Furthermore, Rydberg atom formation is incomplete at the time when the plasma expansion velocity has reached its maximum value. Various effects are under consideration as possible remedies to these inconsistencies.