Evolution dynamics of a dense frozen Rydberg gas to plasma

Abstract

Dense samples of cold Rydberg atoms have previously been observed to spontaneously evolve to a plasma, despite the fact that each atom may be bound by as much as 100 cm −1 . Initially, ionization is caused by blackbody photoionization and Rydberg-Rydberg collisions. After the first electrons leave the interaction region, the net positive charge traps subsequent electrons. As a result, rapid ionization starts to occur after 1 s caused by electron-Rydberg collisions. The resulting cold plasma expands slowly and persists for tens of microseconds. While the initial report on this process identified the key issues described above, it failed to resolve one key aspect of the evolution process. Specifically, redistribution of population to Rydberg states other than the one initially populated was not observed, a necessary mechanism to maintain the energy balance in the system. Here we report new and expanded observations showing such redistribution and confirming theoretical predictions concerning the evolution to a plasma. These measurements also indicate that, for high n states of purely cold Rydberg samples, the initial ionization process which leads to electron trapping is one involving the interactions between Rydberg atoms.

External Control of Electron Temperature in Ultra-Cold Plasmas

“…[1], and in our labs at Colby College. Given that each atom is bound by as much as 100 cm −1 , considerations of energy conservation must be raised: What mechanisms supply the energy for ionization?…”

“…al. [1] set gated integrators over the plasma electron Figure 2.6: The pulsed-amplified CW laser system. The 960 nm, 5mW output of the ECDL is directed through an optical isolator and incident on three LDS925 diode cells where it is amplified, pumped by the second harmonic 532nm source from a Nd: YAG 10ns pulse laser.…”

“…plasma found in fluorescent light tubes, the Sun, and fusion reactors). For most plasmas, the lowest possible temperature is that corresponding to electron energies that lead to the ionization of neutral atoms and therefore the replenishing of ions and electrons lost to the vacuum chamber walls [1]. The energies that yield ionization on impact are on the order of 5 eV (55,000 K).…”

“…Our ultra-cold plasma is created by photoionization of a sample of trapped 85 Rb atoms in a magneto-optical trap (MOT) using a pulsed dye-amplified diode laser. The initial observation of this used 50 µK Xe atoms, but the results have been reproduced using cold Rb, Cs, and Sr atoms [1,2,3]. To a good approximation, the initial electron temperature is obtained from the remaining photon energies in excess of the ionization threshold of 85 Rb; and the initial positive ions maintain the same temperature as the initially trapped atoms [1,2,3].…”

Measurement and simulation of laser-induced fluorescence from nonequilibrium ultracold neutral plasmas

“…The electron temperature can be tuned from approximately 0.5 K to 1000 K. At low initial electron temperature, space-charge effects prevent electrons from leaving, and the plasmas are charge neutral [1]. The electrons screen the ion-ion interactions, playing an important role in the rate at which the ions thermalize the the plasma evolves [5,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29].…”