Properties of the ultracold molecular plasma formed in a seeded supersonic beam of NO

We have prepared an ultracold plasma of NO$^{+}$ molecular cations and electrons, entrained in a seeded supersonic molecular beam as a 1 mm$^{3}$ volume element with a charge density exceeding 10$^{12}$~cm$^{-3}$. Crossed laser beams defining this volume element produce a dense gas of NO molecules excited to a single rovibrationally selected $nf$ Rydberg state. Rydberg-Rydberg Penning interactions initiate the evolution to a plasma on a 100 ns timescale. The first of these electrons escape, after which Rydberg molecules and electrons \--- now trapped in the potential well formed by the macroscopic space charge \--- undergo an avalanche of ionizing collisions. The reservoir of Rydberg binding energy appears to moderate free electron temperature, and the high charge density acts to suppress exothermic three-body recombination. We have measured a rate of plasma expansion over 30 $\mu$s that accords with the Vlasov equations for a quasi-neutral plasma with an electron temperature that falls from an initial 8 K to 1 K, corresponding to an electron correlation, $\Gamma_{e}$, as high as 10.