Recombination Fluorescence in Ultracold Neutral Plasmas

Abstract

We present the first measurements and simulations of recombination fluorescence in ultracold neutral plasmas. In contrast with previous work, experiment and simulation are in significant disagreement. Comparison with a recombination model suggests that the disagreement could be due to the high energy portion of the electron energy distribution or to large energy changes in electron/Rydberg scattering. Recombination fluorescence opens a new diagnostic window in ultracold plasmas because it probes the deeply-bound Rydberg levels, which depend critically on electron energetics.PACS numbers: 34.80. Lx, 52.27.Gr, 52.27.Cm Ultracold neutral plasmas can provide new insights into relaxation phenomena in strongly coupled Coulomb systems. They are created by photoionizing laser cooled atoms and therefore have very precisely controlled initial temperatures and densities [1,2,3,4]. Photoionization causes an impulsive hardening of the interparticle potential [5], and the pathway to (non)equilibrium can be studied in detail [6,7].The electron system equilibrates on the shortest time scales and largely determines how the plasma expands [8,9,10,11,12]. Indirect measurements of the electron temperature agree well with simulations [9,13,14]. Disorder-induced heating, three body recombination (TBR), and electron/Rydberg scattering are important heating mechanisms in the plasma. The latter two are thought to occur on times that are long compared to the electron plasma frequency. Thus TBR can probe the electron system at early times in the plasma evolution after disorder-induced heating has occurred [15].In higher temperature plasmas, the TBR picture is an electron and ion colliding in the presence of another electron. The TRB rate is the two-body collision frequency (nσv th ) multiplied by the probability that a third body is one collision distance away (nb 3 ). The collision distance and cross section depend on the electron temperature (b = e 2 /4πǫ 0 k B T and σ = πb 2 ), giving the well-known TBR rate α 3 = Rn 2 T −9/2 , where R is the TBR rate coefficient [16].In a strongly-coupled system, this binary collision picture breaks down.When the nearest-neighbor potential energy exceeds the kinetic energy (Γ ≡ e 2 n 1/3 /4πǫ 0 k B T > 1), the plasma is strongly coupled. The average distance between electrons becomes comparable to the collision distance b ∼ n −1/3 . The electrons are in a constant collision and TBR becomes, in a sense, many-body recombination. A numerical simulation of early-time recombination suggest that even moderate initial coupling in the electron system changes the recombination rate by perhaps a factor of two [17]. Other theoretical treatments suggest larger changes [18,19]. However, there is no experimental evidence that TBR departs from the standard formulas [20].In this paper we present a new study of three-body recombination in ultracold neutral plasmas. We measure the time-dependent fluorescence emitted by the plasma following electron/ion recombination for a range of initial plasma densities and electr...

Limit of strong ion coupling due to electron shielding

“…These plasmas are typically generated by photo-ionizing lasercooled gases [1] or gases in an ultrasonic jet [2]. They are diagnosed using three-body recombination [3][4][5][6][7][8], thermalization rates [9][10][11][12], electron evaporation or rf absorption [3,[13][14][15][16][17][18], charged particle imaging and detection [2,19], and optical fluorescence [12,20,21] and absorption [22][23][24]. Theoretical calculations and simulations [25][26][27][28][29][30][31][32] give great insights into the properties of these plasmas.…”

“…Collisional processes that are well-understood in terms of interactions between discrete atoms or ions in the plasma, such as three-body recombination, should take on a many-body nature. While evidence for this is seen for dense Rydberg gases [33], only limited evidence for * scott.bergeson@byu.edu departures from traditional plasma physics has been reported in the literature (see, for example [5,10]). …”

Ultracold neutral plasmas

“…A new treatment [10] of low energy transfer collisions has recently been proposed that modifies threebody recombination rates and is in agreement with the microwave ionization results. Recently, Bergeson and Robicheaux [11] observed the fluorescence associated with Rydberg atoms cascading to the ground state. The results are in some disagreement with a standard recombination model, suggesting either that the high energy tail of the electrons distribution or large energy transfer electron-Rydberg collisions are not properly treated in such a model, so our understanding of the basic process of recombination is still incomplete.…”

Microwave diagnostics of ultracold neutral plasma

“…The diagnostics methods of UNP are mainly developed from some well defined technique of optic probes, such as laser induced fluorescence imaging [4], optical absorption imaging [5] and even recombination fluorescence [6]. The optic probes can excite the fluorescence or be absorbed in the case of ions and Rydberg atoms.…”