Demonstration of NO production in air plasma–metallic surface interaction by broadband laser-induced fluorescence

This paper deals with high temperature chemistry of air in interaction with a metallic surface. A subsonic air plasma is created with an inductive torch and is investigated using broadband KrF laser-induced fluorescence. Nitrogen monoxide fluorescence spectra are recorded in the free subsonic plasma jet as well as within the boundary layer above a stainless steel flat plate water-cooled to 300 K. The comparison between calculated and experimental calibrated fluorescence spectra allows the determination of rotational and vibrational temperatures as well as densities of NO ground state. The results demonstrate a strong non-equilibrium between rotational and vibrational temperatures in both the free jet and the boundary layer including very close to the wall. Density determinations show that nitrogen monoxide is in chemical equilibrium on the axis of the free jet but not on its boundaries. The NO results are analysed together with previous O2 and N2 results obtained by Raman spectroscopy in order to explain the increasing NO densities observed within the boundary layer. The discussion highlights a double production of NO due to catalytic reactions at the wall and to the exothermic reaction N + O2 → NO + O within the boundary layer following O2 recombination at the wall.