The very slow expansion of an ultracold plasma formed in a seeded supersonic molecular beam of NO

TL;DR

This study investigates the slow expansion of a cold ultracold plasma formed from nitric oxide in a supersonic molecular beam, measuring its expansion rate and comparing it to theoretical models.

Contribution

It provides the first measurement of the expansion rate of a NO ultracold plasma and confirms agreement with Vlasov equation predictions for electron temperature.

Findings

Plasma expands slowly over time.

Expansion rate matches Vlasov model predictions.

Electron temperature estimated at approximately 8 K.

Abstract

The double-resonant laser excitation of nitric oxide, cooled to 1 K in a seeded supersonic molecular beam, yields a gas of $\approx10^{12}$ molecules cm$^{-3}$ in a single selected Ryberg state. This population evolves to produce prompt free electrons and a durable cold plasma of electrons and intact NO$^{+}$ ions. This plasma travels with the molecular beam through a field free region to encounter a grid. The atomic weight of the expansion gas controls the beam velocity and hence the flight time from the interaction region to the grid. Monitoring electron production as the plasma traverses this grid measures its longitudinal width as a function of flight time. Comparing these widths to the width of the laser beam that defines the initial size of the illuminated volume allows us to gauge the rate of expansion of the plasma. We find that the plasma created from the evolution of a Rydberg gas of NO expands at a small but measurable rate, and that this rate of expansion accords with the Vlasov equations for an initial electron temperature of $T_{e} \approx 8 K$.