Expansion of an ultracold Rydberg plasma

TL;DR

This study combines experimental measurements and numerical simulations to analyze the expansion velocities and electron temperatures of ultra-cold Rydberg plasmas, revealing how initial conditions and Rydberg states influence plasma properties.

Contribution

It provides new insights into the dependence of electron temperature on Rydberg atom density, binding energy, and ionization fraction, with systematic experimental and numerical analysis.

Findings

Electron temperature $T_{e,0}$ is insensitive to ionization mechanism for $n > 40$.

$k_B T_{e,0}$ correlates with the fraction of ionized atoms and scales with $|E_{b,i}|$.

Plasmas from $n extless 40$ Rydberg states show no significant additional ionization after threshold.

Abstract

We report a systematic experimental and numerical study of the expansion of ultra-cold Rydberg plasmas. Specifically, we have measured the asymptotic expansion velocities, $v_0$, of ultra-cold neutral plasmas (UNPs) which evolve from cold, dense samples of Rydberg rubidium atoms using ion time-of-flight spectroscopy. From this, we have obtained values for the effective initial plasma electron temperature, $T_{e,0} = m_{ion} v_0^2/k_B$ (where $m_{ion}$ is the Rb$^+$ ion mass), as a function of the original Rydberg atom density and binding energy, $E_{b,i}$. We have also simulated numerically the interaction of UNPs with a large reservoir of Rydberg atoms to obtain data to compare with our experimental results. We find that for Rydberg atom densities in the range $10^7 - 10^9$ cm$^{-3}$, for states with principal quantum number $n > 40$, $T_{e,0}$ is insensitive to the initial ionization mechanism which seeds the plasma. In addition, the quantity $k_B \, T_{e,0}$ is strongly correlated with the fraction of atoms which ionize, and is in the range $0.6 \times |E_{b,i}| \lesssim k_BT_{e,0} \lesssim 2.5 \times |E_{b,i}|$. On the other hand, plasmas from Rydberg samples with $n \lesssim 40$ evolve with no significant additional ionization of the remaining atoms once a threshold number of ions has been established. The dominant interaction between the plasma electrons and the Rydberg atoms is one in which the atoms are deexcited, a heating process for electrons that competes with adiabatic cooling to establish an equilibrium where $T_{e,0}$ is determined by their Coulomb coupling parameter, $\Gamma_e \sim 0.01$.