Recombination Fluorescence in Ultracold Neutral Plasmas

We present the first measurements and simulations of recombination fluorescence from ultracold neutral calcium plasmas. This method probes three-body recombination at times less than $1\mu \mathrm{s}$, shorter than previously published time scales. For the lowest initial electron temperatures, the recombination rate scales with the density as $n_0^{2.2}$, significantly slower than the predicted $n_0^3$. Recombination fluorescence opens a new diagnostic window in ultracold plasmas. In most cases it probes deeply bound level populations that depend critically on electron energetics. However, a perturbation in the calcium $4snd$ Rydberg series allows our fluorescence measurements to probe the population in weakly bound levels that result just after recombination.

Figure 1

Recombination fluorescence signal at 410 nm. The time at which the fluorescence signal peaks depends on the density, with higher density plasma signals peaking earlier than low density plasmas. At $1\mu \mathrm{s}$, the plasma density has changed by 10% due to expansion. Inset: Partial energy level diagram for neutral calcium.

Figure 2

A comparison of the simulation and experiment. For $T=30$ and 57 K, the simulated and experimental fluorescence signals both scale with $n_0^3$ [see Eq. (3)]. The simulation plots are for densities of 5, 10, and $20\times {10}^9{\mathrm{cm}}^{-3}$, corresponding to nonequilibrium initial values of $\Gamma =0.08$, 0.10, and 0.13 for $T=57\mathrm{K}$ and $\Gamma =0.15$, 0.19, and 0.24 for $T=30\mathrm{K}$. The densities in the experimental plots cover the same range. White, gray, and black circles plot the low, medium, and high densities. For this figure, the fluorescence signal is divided by density raised to the indicated power, and each set of plots is scaled for comparison. The 100–200 ns delayed onset of the experimental signal probably indicates the time required for the electrons to fill out a Boltzmann distribution.

Figure 3

The slope of the initial recombination fluorescence signal as a function of initial electron temperature and density. $T_e=57\mathrm{K}$ (black upside-down triangles), $T_e=30\mathrm{K}$ (circles), and $T_e=0.5\mathrm{K}$ (black triangles). The higher temperature signals scale as $n_0^3$, in agreement with Eq. (3). The lowest temperature scales as $n_0^{2.2}$, contrary to Eq. (4).