Molecular ion–electron recombination in an expanding ultracold neutral plasma of NO+

Abstract

Using state-selected double-resonant excitation, we create a Rydberg gas of NO molecules excited to the principal quantum number n = 50 of the f-series converging to the ion rotational level, N(+) = 2. This gas evolves to form an ultracold plasma, which expands under the thermal pressure of its electrons, and dissipates by electron-ion recombination. Under conditions chosen for this experiment, the observed rates of expansion vary with selected plasma density. Electron temperatures derived from these expansion rates vary from T(e) = 12 K for the highest density up to 16 K at four-fold lower density. Over this range, the apparent electron coupling parameter, defined as Γ(e) = e(2)/4πε(0)ak(B)T(e), falls from nearly three to about one. The decay of charged-particle density fits with a kinetic model that includes parallel paths of direct two-body and stepwise three-body dissociative recombination. The overall recombinative decay follows a second-order rate law, with an observed rate constant that fits with established scattering-theory estimates for elementary two-body dissociative recombination. A small residual increase in this rate constant with decreasing charged-particle density suggests a growing importance of the three-body recombination channel under conditions of decreasing electron correlation.

Dissociation and the Development of Spatial Correlation in a Molecular Ultracold Plasma

“…In a freely expanding plasma, the ambipolar pressure of the electron gas causes a radial acceleration of the ions [6]. The expansion we observe accords with this mechanism [19,20], and we find that the ions reach a terminal velocity that varies with the initial quantum number selected in the preparation of the Rydberg gas. Figure 1 plots terminal velocities of plasma expansion as a function of n 0 .…”

Dynamics of colliding ultracold plasmas

“…The plotted width begins to change slowly, then rises with a slope that reflects a ballistic radial ion velocity of 38 m s -1 . Previous work has shown that such a variation in width as a function of time for a simple laserproduced molecular beam plasma conforms reasonably with the Vlasov equations for a self-similar expansion of a quasineu tral Gaussian distribution of ions and electrons [11,16], much as observed in a MOT [1]:…”

“…Our model makes no provision for the dissociative recombination (DR) of NO+ with electrons [ 16,19], or for that matter, three-body recombination of ions and electrons, which can be expected to limit the density of shock structures in atomic as well as molecular plasmas.…”

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.…”