Two-temperature Collisional-radiative Modeling of Partially Ionized O₂-Ar Mixtures over 8000-10,000 K Behind...

The collisional excitation kinetics of atomic oxygen was studied behind reflected shock waves using tunable diode laser absorption spectroscopy. A test gas mixture of 1% O₂/Ar was shock-heated to temperatures between 8000 and 10,000 K and pressures between 0.15 and 1 atm. The time evolution of the atomic oxygen population in the 3 s ⁵S⁰ state was monitored by laser absorption at 777.2 nm. The measured O(3 s ⁵S⁰) population revealed multistage behavior that was not observed in previous measurements over a temperature range of 5300-7200 K. To interpret the multistage behavior, a three-level collisional-radiative model for atomic oxygen excitation kinetics was developed. The model utilized two independent temperatures, that is, heavy particle translational temperature T_tr and electron translational temperature T_e, to describe the fundamental rate constants of atomic oxygen excitation because of collisions with heavy particles and electrons, respectively. The heavy particle excitation rate was inferred from the early stage of the measurement to be k(³P →⁵S⁰) = 3.4 × 10^-27 (T/K)^0.5(1.061 × 10⁵ + 2 (T/K)) exp(-1.061 × 10⁵ K/T) ± 50% m³ s^-1. The electron impact excitation rate constant of oxygen, electron impact, and heavy particle impact ionization rate constants of Argon were modified in the model to match the experimental population time histories. The modified rate parameters are also reported for the temperature range explored in the current study.