  The double-resonant laser excitation of nitric oxide, cooled to 1 K in a
seeded supersonic molecular beam, yields a gas of $\approx10^{12}$ molecules
cm$^{-3}$ in a single selected Ryberg state. This population evolves to produce
prompt free electrons and a durable cold plasma of electrons and intact
NO$^{+}$ ions. This plasma travels with the molecular beam through a field free
region to encounter a grid. The atomic weight of the expansion gas controls the
beam velocity and hence the flight time from the interaction region to the
grid. Monitoring electron production as the plasma traverses this grid measures
its longitudinal width as a function of flight time. Comparing these widths to
the width of the laser beam that defines the initial size of the illuminated
volume allows us to gauge the rate of expansion of the plasma. We find that the
plasma created from the evolution of a Rydberg gas of NO expands at a small but
measurable rate, and that this rate of expansion accords with the Vlasov
equations for an initial electron temperature of $T_{e} \approx 8 K$.
