Evolution from a Molecular Rydberg Gas to an Ultracold Plasma in a Seeded Supersonic Expansion of NO

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 single Rydberg states extending as deep as $80{\mathrm{cm}}^{-1}$ below the lowest ionization threshold. The density of excited molecules in the illuminated volume approaches $1\times {10}^{13}{\mathrm{cm}}^{-3}$. This population evolves to produce free electrons and a durable cold plasma of electrons and intact ${\mathrm{NO}}^{+}$ ions.

Figure 1

Schematic diagram showing the molecular beam flight path from a differentially pumped source chamber through a skimmer to enter a system of three grids ending in a flight tube capped by a microchannel plate detector. Approximately to scale with the $G_1$ to $G_2$ spacing of 2 cm.

Figure 2

Oscilloscope trace showing the arrival time of electrons produced following two-color production of NO molecules in the $52f(2)$ Rydberg state with $-250\mathrm{mV}$ applied to $G_1$ and 60 V applied to $G_3$.

Figure 3

High-Rydberg excitation spectra of the plasma signal under conditions in which $1\mu \mathrm{s}$ positive pulses at voltages producing the electrostatic fields indicated are applied starting 500 ns after ${\omega }_2$. Diamonds show the adiabatic field-ionization threshold for the principal quantum numbers from 40 to 110.