The effect of Rydberg atoms on electron temperature in ultra-cold neutral plasmas

We describe recent developments in our ongoing research in which Rydberg atoms are embedded into an ultra-cold neutral plasma (UNP). The UNP is created with initial electron temperature $T_{e,0}$ by photoionization of rubidium atoms in a MOT. At a controllable time delay (5 ns - 10 $\mu$s), atoms in a specific Rydberg state are embedded in the UNP by a narrow bandwidth pulsed laser. In such a system, it is predicted that the plasma electrons may be cooled if the Rydberg binding energy, $E_b$, is greater than $4k_BT_e$ (see, for example\footnote{T. Pohl {\it et al.}, {\it Eur. Phys. J. D}, {\bf 40}, 45 (2006)}). We have identified an experimental signature that correlates with the plasma electron temperature change, namely, whether the plasma lifetime increases or decreases when Rydbergs are added. The ``crossover'' condition, where the UNP lifetime remains the same when Rydbergs are added, can then be plotted (i.e., $E_b$ vs. $T_{e,0}$ at crossover) and compared with theoretically derived crossover conditions for UNP expansion velocity, electron temperature, etc., using a model derived from the work of Robicheaux and Hansen.\footnote{Robicheaux and Hansen, {\it Phys. Plasmas}, {\bf 10}, 2217 (2003)}