Polarization-sensitive coherent anti-Stokes Raman scattering applied to the detection of NO in a microwave discharge for reduction of NO

A coherent anti-Stokes Raman scattering (CARS) set-up has been developed to study the reduction of nitric oxide (NO) by a microwave-generated nitrogen (N2) plasma under atmospheric pressure. A frequency-doubled Nd:YAG laser provides two pump beams at λ=532 nm and excites a dye laser, which is tunable between 588 and 615 nm. Density and temperature profile measurements of N2 in the cylindrical microwave discharge by the common CARS technique deliver an axis temperature of 7000 K at P = 800 W input power. The detection of a minority of NO in N2 under atmospheric pressure by CARS is limited to a concentration of 2500 ppm. By applying polarization-sensitive CARS the detection limit can be scaled down to 200 ppm. This technique is used to examine the reduction of NO in a reaction chamber fed by vibrationally excited N2 and N entering the chamber through a nozzle. Behind the nozzle most of the NO is decomposed. An overall reduction efficiency for NO of 65-85% was found, decreasing with growing NO concentration. (author)