Comment on "Using Three-Body Recombination to Extract Electron Temperatures of Ultracold Plasmas"

TL;DR

This paper argues that the observed electron temperature decay in ultracold plasmas can be explained by elastic processes and virialization, challenging the previous attribution to inelastic three-body recombination effects.

Contribution

It provides a theoretical explanation for the t^{-1} temperature decay in ultracold plasmas based on elastic processes and electron-ion correlations, supported by molecular-dynamic simulations.

Findings

The t^{-1} temperature decay can be explained by elastic processes.

Virialization of electron velocities accounts for the observed behavior.

Inelastic processes are secondary in influencing the temperature decay.

Abstract

In the recent work Fletcher et al. [Phys. Rev. Lett., v.99, p.145001 (2007)] reported on the novel experimental technique, enabling to measure the temperature of the expanding ultracold plasmas over a considerable time interval (up to 60-70 {\mu}s). It was unexpectedly found that the electron temperature dropped with time as T_e(t) ~ t^{-\alpha} with \alpha = 1.2 +/- 0.1 \approx 1 instead of \alpha = 2, which would be expected for the adiabatic cooling of electrons in the cloud expanding linearly in time. The above-cited authors supposed that 'the difference is likely due to the significant heating effects from 3-body recombination' (i.e. the inelastic processes), but they did not provide sufficient quantitative estimates supporting such a conclusion. The aim of the present comment is to mention that the experimentally revealed t^{-1}-dependence can be explained under quite general assumptions by the purely elastic processes in the ultracold plasma, as it was done a few years ago in our work [Yu.V. Dumin, J. Low Temp. Phys., v.119, p.377 (2000)]. Therefore, t^{-1}-dependence revealed in the above-cited experiment should be primarily a manifestation of "virialization" of the electron velocities in the regime of strong electron-ion correlations, while the heat release by inelastic processes should be of secondary importance (it might be responsible, particularly, for changing the exponent from -1 to -1.2). This is confirmed also by our ab initio molecular-dynamic simulations taking into account the strong electron-ion correlations (i.e. not based on the PIC method, Vlasov approximation for electrons, etc., which are applicable only to small-angular scattering).