Kinetics of Excited Oxygen Formation in Shock-Heated O2-Ar Mixtures

The formation of electronically excited atomic oxygen was studied behind reflected shock waves using cavity-enhanced absorption spectroscopy. Mixtures of 1% O2-Ar were shock-heated to 5400-7500 K, and two distributed-feedback diode lasers near 777.2 and 844.6 nm were used to measure time-resolved populations of atomic oxygen's 5S?? and 3S?? electronic states, respectively. Measurements were compared with simulated population time histories obtained using two different kinetic models that accounted for thermal nonequilibrium effects: (1) a multitemperature model and (2) a reduced collisional-radiative model. The former assumed a Boltzmann distribution of electronic energy, whereas the latter allowed for non-Boltzmann populations by treating the probed electronic states as pseudospecies and accounting for dominant electronic excitation/de-excitation processes. The effects of heavy-particle collisions were investigated and found to play a major role in the kinetics of O atom electronic excitation at the conditions studied. For the first time, rate constants (kM) for O atom electronic excitation from the ground state (3P) due to collisions with argon atoms were directly inferred using the reduced collisional-radiative model, kM(3P ??? 5S??) = 7.8 ?? 10-17T0.5 exp( 1.061 ?? 105K/T) ?? 25% cm3 s-1 and kM(3P ??? 3S??) = 2.5 ?? 10-17T0.5 exp( 1.105 ?? 105K/T) ?? 25% cm3 s-1.