A laser-induced fluorescence study of the NO and O densities and their kinetics in a pulsed N2-O2 microwave plasma

Ground state NO(X) and O(2p 4 3 P) kinetics in a fully modulated (50% duty cycle at 500 Hz) surfaguide microwave discharge of 50%-50% N2-O2 gas mixture were studied using laser-induced fluorescence (LIF) and two-photon absorption LIF (TALIF), respectively. The measurements were performed at a constant average applied power of 0.7 kW, for two gas pressures: 2 and 5 Torr. The kinetics of the NO(A) state using the emission band with the band head at 247.87 nm ( A 2 Σ + , v ′ = 0 → X 2 Π , v ′ ′ = 2 ) and O(3p 3P) using the 2p33p 3 P → 3 s 3S emission line at 844.64 nm were also studied. A zero-dimensional (0D) kinetic model was used to compare with the experimental findings and determine the important reaction mechanisms behind the kinetic behavior. We observed that NO(X) reacting with O atoms to form NO2 lowers the initial NO(X) density over the plasma on-time. O ground state atoms follow a similar pattern of decaying during the on-time and recovering during the off-time. Finally, NO2 dissociates to reform NO(X) in the off-time and the post-discharge. In contrast, the NO(A) density increases in the on-time and decays in the off-time. This study aimed to compare traditional methods (LIF/TALIF and actinometry) for accurately determining the absolute densities and kinetics of NO(X) and O(2p 4 3 P) atoms, and to investigate the kinetics of key species, including their excited states.