| The double-resonant laser excitation of nitric oxide, cooled to 1 K in a seeded supersonic molecular beam, yields a gas of ≈1012 molecules cm-3 in a single selected Ryberg state e.g. 52f(2), or n=52 in the f series converging to NO+ rotational state, N+=2. This population evolves to produce prompt free electrons and a durable cold quasineutral plasma of electrons and intact NO+ ions. Thereafter, a field of amplitude as low as 5 V/cm, far smaller than that required to field-ionize n=52, extracts a small signal of electrons, but pulses of amplitude even as high as 200 V/cm, are insufficient to destroy the plasma. These observations signal two properties that confirm the evolution to a cold plasma: the presence of a surface charge of electrons, extractable by a weak field, and a Debye screening length much smaller than the diameter of the plasma, which shields the charged particles in its core. Downstream, transmission through a grid samples the electrons in this core. The plasma expands at a small but measurable rate according with the Vlasov equations for an initial electron temperature of T≈ 7~K. Combining experimental estimates of the magnitude of the plasma charge and the number density of ion, we have developed a simple thermochemical model that explains the evolution of the plasma to an ultracold electron temperature. |
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