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 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.