Formation of Rydberg atoms in an expanding ultracold neutral plasma
Michael Lim, Thomas Killian, Simone Kulin, Rainer Dumke, Scott
Bergeson, and Steve Rolston
We create an ultracold neutral plasma by photoionizing laser-cooled
metastable xenon atoms. By changing the frequency and intensity of the
ionizing laser pulses, we vary the initial temperature (1-1000 K)
and density (105-1010 cm-3) of the resulting
plasma. In the first 100 ms, while the
plasma is rapidly expanding, up to 20% of the free charges recombine into
Rydberg atoms. The binding energy released in this process approximately
equals the increase in the kinetic energy of the remaining free electrons
that was observed in a previous experiment. Although three-body recombination
is expected to dominate in this regime, most of our results are inconsistent
with this mechanism. Liquid-like spatial correlations in our strongly
coupled plasmas may render a many-body process more important than conventional
three-body recombination.