Absolute densities of H and OH in spatial afterglow of atmospheric-pressure H₂O/Ar plasma

We measured the density distributions of H and OH in the jet part of an atmospheric-pressure H₂O/Ar plasma with the intention of applying it to an atomic hydrogen source in catalytic synthesis of ammonia. The H density was measured by two-photon absorption laser induced fluorescence with the absolute calibration using Kr. The OH density was measured by laser-induced fluorescence spectroscopy, where the absolute OH density was estimated based on the spatial distribution and the rate coefficient of OH+OH+M→H₂O₂+M. When the flow rates of Ar and H₂O were 2.8 and 0.034 L/min, respectively, the H density at the jet nozzle was approximately 1×10¹⁵ cm^-3, while the OH density at the same position was 1×10¹⁴ cm^-3. The higher H density than the OH density is preferable if we apply this plasma source to the ammonia synthesis. The decay of the H density was steeper than that of the OH density, and as a result, similar H and OH densities were observed at a distance of 2 cm from the nozzle. The steeper decay of the H density contradicts the prediction based on the rate coefficients of OH+OH+M→H₂O₂+M and H+H+M→H₂+M.