A pulse-pileup correction procedure for rapid measurements of hydroxyl concentrations using picosecond time-resolved laser-induced fluorescence

Abstract

The measurement of fluorescence lifetimes is important for determining minor-species concentrations in flames when using linear laser-induced fluorescence (LIF). Applications of LIF to turbulent flames require that the quenching rate coefficient be determined in less than ∼100 μs. Moreover, the measurement technique must be insensitive to the existence of relatively large backgrounds, such as occur from flame emission. To meet these goals, we have recently developed a rapid, gated photon-counting technique, termed LIFTIME. However, for ultimate application to turbulent time-series measurements, LIFTIME must be extended to photon count rates which unfortunately result in nonlinear discriminator operation. In this paper, a correction technique is derived to permit quantitative measurements of fluorescence lifetimes and concentrations at sampling rates up to 4 kHz. The technique was tested against liquid samples having a known lifetime and is further shown to reproduce previous hydroxyl concentration measurements in a series of laminar flames with total photon count rates of up to ∼35 million detected photoelectrons per second. The fluorescence lifetimes and hydroxyl concentrations are shown to be measured with ∼10% accuracy (68% confidence interval) for sampling times as low as 250 μs.