We have used the free expansion of ultracold neutral plasmas as atime-resolved probe of electron temperature. A combination of experimentalmeasurements of the ion expansion velocity and numerical simulationscharacterize the crossover from an elastic-collision regime at low initialGamma_e, which is dominated by adiabatic cooling of the electrons, to theregime of high Gamma_e in which inelastic processes drastically heat theelectrons. We identify the time scales and relative contributions of variousprocesses, and experimentally show the importance of radiative decay anddisorder-induced electron heating for the first time in ultracold neutralplasmas.
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