Properties of the ultracold molecular plasma formed in a supersonic beam of nitric oxide
The Wall Hour
02 Apr 2009
Talk by Ed Grant, Professor and Chair, Chemistry, University of British Columbia
When excited to within about 100 cm^-1 of the ionization threshold, ensembles of ultracold Rydberg atoms evolve to form an ultracold plasma. Such systems, in which the driving force of many-body electrostatic interactions can substantially exceed thermal energies, have attracted a great deal of attention as laboratories for the study of dissipation-relaxation in quasi-condensed-phase, strongly correlated systems. The requirement for laser cooling in magneto-optical traps has limited the range of conventional precursor systems to alkali metal, alkaline earth and rare gas atoms. We report the spontaneous formation of a plasma from a gas of cold Rydberg molecules. Double-resonant laser excitation promotes nitric oxide, cooled to 1 K in a seeded supersonic molecular beam, to produce a dense gas of NO molecules excited to a single rovibrationally selected nf Rydberg state. This population evolves on a 100 ns timescale to form free electrons and a durable cold plasma of electrons and intact NO+ ions at a charge density exceeding 10^12 cm^-3. 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 µs 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 as high as 10.