This paper describes a method that combines a laser ignition technique with laser-induced fluorescence (LIF) spectroscopy for studying the gas- phase products in a laser-induced subignition zone and the reactions that lead to a self-sustained ignition. The experiment comprises a tunable 180 W CO2-laser as ignition source, an excimer pumped dye laser for inducing the fluorescence, and a spectrometer equipped with an optical multichannel analyzer. This technique was used for measurements of relative NO and CN concentrations in the subignition zone of RDX (1,3,5-Trinitrohexahydro-s- triazine) in pseudo-real time (time resolution better than 1 μs). By using LIF technique for measuring the relative population of different vibrational levels, we were able to calculate the vibrational temperature in the gas phase reaction zone in front of the sample at subignition to approximately 3100 K. The measurements show clearly that the chemical reactions and the diffusion in the subignition zone play an important part long before a self- sustained reaction occurs, and thus influence the sensitivity of an explosive. By using LIF imaging technique, two-dimensional images of the NO concentration were registered at different times in the ignition pulse, and the wavelength dependence of the ignition source was also studied. The results correspond to a model for fast radiative ignition where Lambert-Beer absorption is the main energy interaction mechanism between the energetic material and the laser beam.
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