  We have measured the asymptotic expansion velocities of ultra-cold plasmas
(UNPs) which evolve from cold, dense, samples of Rydberg rubidium atoms using
ion time-of-flight spectroscopy. From this, we have obtained values for the
initial plasma electron temperature as functions of the original Rydberg atom
density and binding energy. Our results show that the electron temperature is
determined principally by the plasma environment when the UNP decouples from
the Rydberg atoms, which occurs when the plasma electrons become too cold to
ionize the remaining Rydberg population. Furthermore, the dependence of the
electron temperature on Rydberg atom density gives strong indirect evidence for
the existence of a bottleneck in the spectrum of Rydberg states that coexist
with a cold plasma.
