AMO seminar

Title: Ultracold plasma fission under Rydberg-gas quantum-state control
Speaker: Ed Grant,  Department of Chemistry, University of British Columbia, Canada

Time: Thursday April 23, 15:15
Duration: 45 minutes
Place: 1520-732
Coffee, tea and cake will be served at 15:01

Abstract

The state of strong coupling plays an important role in the charge transport properties of complex systems ranging in size from nanoparticles to globular star clusters. Ultracold plasmas afford a means to study strongly coupled systems with quantum state control in a laboratory setting.

This talk will describe the behaviour of the ellipsoidal molecular ultracold plasma that evolves from a state-selected nitric oxide Rydberg gas in a seeded supersonic molecular beam. In one type of experiment, we propagate this plasma 600 mm in z to strike an imaging detector that records the charge distribution in x and y. Detected images detail the evolution of charged-particle density as a function of selected Rydberg gas initial principal quantum number, n₀ and Rydberg gas density, ?₀. We find that conditions of high n₀ or high ?₀ systematically break the ellipsoidal symmetry of plasma expansion.

FIG. 1: x; y plasma images collected after a 600 mm flight path for a 58 f(2) Rydberg gas of NO, prepared with varying ?₁ laser pulse energies from 1.75 ?J to 4.24 ?J.

This symmetry breaking occurs when Penning ionization and avalanche in the core of the ellipsoid leads plasma formation in the wings. Ions accelerated by electron-gas expansion in ±x transfer momentum to Rydberg molecules. As the plasma develops, the expanding gas of hot electrons pushes beyond the ions, causing exterior conditions to deviate from quasi-neutrality. The lowered potential energy of the resulting double-layer acts as a surface tension. When the electron kinetic energy transferred to ion motion in the opposing elements of plasma volume exceeds the surface tension about x = 0, the ellipsoid spontaneously separates to form volumes with smaller surface area and higher stability, much like the separation of a rotating liquid drop, or the fission of a metal cluster.

Overall, this process transforms electron thermal energy, first into radial motion of electrons and ions, then into the recoil energy of plasma lobes. The conversion of electron thermal energy into mesoscopic recoil reduces the local ion and electron temperature within the moving plasma volumes, which then exhibit a slow expansion characteristic of strongly coupled ions and electrons.

Henrik Stapelfeldt