Collisional excitation kinetics for O(3s^{5}S^{o}) and O(3p^{5}P_{3}) states using laser absorption spectroscopy in shock-heated weakly ionized O_{2}-Ar mixture.

Abstract

Collisional excitation kinetics for atomic oxygen is studied behind reflected shock waves in 1%O_{2}/Ar mixtures over 10\u2009000-11\xa0000K using laser absorption spectroscopy of the O(3s^{5}S^{o}) to O(3p^{5}P_{3}) transition at 777\xa0nm and the O(3p^{5}P_{3}) to O(3d\xa0^{5}D_{2,3,4}^{o}) transitions at 926\xa0nm. Four time histories are inferred simultaneously from the absorbance of the two transitions: the population density of level 4 of atomic oxygen, i.e., the O(3s\xa0^{5}S^{o}) state, n_{4}; the population density of level 6 of atomic oxygen, i.e., the O(3p^{5}P_{3}) state, n_{6}; the electron number density, n_{e}; and the heavy-particle translational temperature, T_{tr}. Atomic oxygen in the levels 4 and 6 are not in equilibrium with the ground-state atomic oxygen as the measurements of n_{4} and n_{6} are generally 3-20 times smaller than the corresponding values under Boltzmann equilibrium at T_{tr}. However, these two states are close to partial equilibrium with each other within the test time, indicating strong heavy-particle cross coupling between levels 4 and 6 of atomic oxygen. A simplified two-temperature collisional-radiative (CR) model is developed to study the thermal and chemical nonequilibrium of atomic oxygen following shock heating. The four measured time histories are used to optimize the 12 collisional rate constants in the CR model using a stochastic gradient descent (SGD) algorithm. The time-history results, diagnostic methods, and collisional-radiative model presented in the current study are potentially useful in studies of high-enthalpy air, plasma processing, or other applications involving weakly ionized oxygen.