  A Rydberg gas of NO entrained in a supersonic molecular beam releases
electrons as it evolves to form an ultracold plasma. The size of this signal,
compared with that extracted by the subsequent application of a pulsed electric
field, determines the absolute magnitude of the plasma charge. This
information, combined with the number density of ions, supports a simple
thermochemical model that explains the evolution of the plasma to an ultracold
electron temperature.
