Pulsed photoexcitation of hydrocarbon fuels doped with organic molecules exhibits a temperature-dependent fluorescence spectrum that is used as the basis for a weakly intrusive optical thermometer. By use of pulsed excitation from a 308-nm 8-ns XeCl excimer laser with gated detection of the fluorescence emissions from doped -heptane, we demonstrate that time-resolved measurement of the excited monomer and the redshifted excited-state complex (exciplex) fluorescence emissions can yield sub-1° accuracy for temperatures ranging from 440 K to the vicinity of the critical temperature (540 K). The experiments also show that the exciplex fluorescence spectrum is pressure independent below and above supercritical pressure.
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