  We report measurements of the ion velocity distribution in an ultracold
neutral plasma derived from a dense, cold Rydberg gas in a MOT. The Rydberg
atoms are excited using a resonant two-step excitation pathway with lasers of 4
ns duration. The plasma forms spontaneously and rapidly. The rms width of the
ion velocity distribution is determined by measuring laser-induced fluorescence
(LIF) of the ions. The measured excitation efficiency is compared with a
Monte-Carlo wavefunction calculation, and significant differences are observed.
We discuss the conditions for blockaded Rydberg excitation and the subsequent
spatial ordering of Rydberg atom domains. While the blockade interaction is
greater than the Rabi frequency in portions of the atomic sample, no evidence
for spatial ordering is observed.
