Demonstration of a two-photon, confocal laser-induced fluorescence technique for the detection of nitric oxide in atmospheric pressure flows and hydrocarbon-air flames

The detection and measurement of levels of nitric oxide (NO) is of importance in combustion science, environmental pollution and biology. Here we describe a two-photon laser-induced fluorescence scheme for the detection of NO in an atmospheric pressure flow and a hydrocarbon-air flame. Excitation from a simple, YAG-pumped, Coumarin 450 dye system near 452.6 nm was used to promote the two-photon NO (A (2)Sigma(+), upsilon' = 0 <-- X (2)Pi, upsilon " = 0) transition in the gamma(0, 0) band. Subsequent fluorescence detection in the range 200-300 nm permitted almost total rejection of elastic and geometrical scattering of laser radiation for excellent signal-to-noise characteristics. The use of a confocal optical arrangement for single-point measurements of ppm-level NO concentrations permits applications offering only a single direction far optical access and has potential for further development using fibre-optical techniques.