Relaxation of Rydberg states in an ultracold plasma

The variations in the population densities of Rydberg states (with principal quantum number n ~50 – 100) in an expanding ultracold laser plasma (N_e ~10⁹ – 10⁴ cm^-3, T_e ~10 – 0.5 K) are calculated. The electron temperature measurement from the population densities of Rydberg states (T C Killian et al., Phys. Rev. Lett., vol. 86, p. 3759, 2001) is shown to be described by the conventional three-body recombination theory with the inclusion of recombination heating of free electrons.