Laser Induced Fluorescence Studies of NO Kinetics in Short Pulse Air and Air-Fuel Nonequilibrium Discharges

Laser Induced Fluorescence is used to measure absolute NO concentrations in air, methane-air, and ethylene-air non-equilibrium plasmas, as a function of time after initiation of a single 25 nsec discharge pulse. Peak NO density in air at 60 torr is $\sim $8.10$^{12}$ cm$^{-3}$ occurring at $\sim $500 $\mu $s after the pulse, with decay time of $\sim $16.5 msec. Peak NO atom mole fraction in methane-air at $\phi $=0.5 is approximately equal to that in pure air with similar rise and decay rate. In $\phi $ = 0.5 ethylene-air, the rise and decay times are also comparable to air and methane--air, but peak NO concentration is a factor of $\sim $2.5 lower. Spontaneous emission measurements show that N$_{2}$(C) and NO (A) decay in $\sim $25ns and $\sim $2.5$\mu $s, respectively. Kinetic modeling calculations incorporating Boltzmann solver for EEDF, and electron impact and full air species kinetics, complemented with the GRI Mech 3.0 hydrocarbon oxidation mechanism, are compared with the experimental data using three different mechanisms. It is concluded that processes involving long lifetime ($\sim $100 $\mu $sec) meta-stable states, such as N$_{2}$ (X,v) and O$_{2}$(b$^{1}\Sigma )$, which are formed by quenching of the metastable N$_{2}$ (A) state by ground state O$_{2}$, play a dominant role in NO formation.