  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.
