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 (k_M) 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, k_M(³P → ⁵S°) = 7.8 × 10^-17T^0.5 exp(-1.061 × 10⁵K/T) ± 25% cm³ s^-1 and k_M(³P → ³S°) = 2.5 × 10^-17T^0.5 exp(-1.105 × 10⁵K/T) ± 25% cm³ s^-1.