High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide. I. Argon dilution from 2200 to 8700 K

This work investigates the high-temperature vibrational relaxation and decomposition of nitric oxide (NO) diluted in argon (Ar) to target NO–Ar and NO–NO interactions and to augment the subsequent inference of rates for NO diluted in nitrogen (N₂). [J. W. Streicher et al., “High-temperature vibrational relaxation and decomposition of shock-heated nitric oxide. II. Nitrogen dilution from 1900 to 8200 K,” Phys. Fluids (submitted)]. In both Part I and Part II, two continuous-wave ultraviolet laser diagnostics were used to probe quantum-state-specific time-histories of NO behind reflected shocks in high-temperature shock-tube experiments, enabling inferences of multiple vibrational relaxation times and reaction rate constants for NO decomposition reactions. These diagnostics both probed absorbance (α) in the ground vibrational state of NO but in multiple rotational states utilizing light at 224.8150 and 226.1025 nm. The absorbance was subsequently used to infer quantum-state-specific time-histories for translational/rotational temperature (T_tr) via the absorbance ratio and number density of NO (n_NO) via α, T_tr, and the absorbance cross sections (σ). The experiments for Ar dilution probed mixtures of 2% NO/Ar, 1% NO/Ar, and 0.4% NO/Ar for initial post-reflected-shock conditions from 2200–8700 K and 0.12–0.97 atm. Further analysis of the absorbance, temperature, and number density time-histories yielded two vibrational relaxation times (⁠$τVTNO−Ar$ and $τVTNO−NO$⁠) and four rate coefficients for multiple NO decomposition reactions (⁠$kdNO−Ar$⁠, $kdNO−NO$⁠, $kfN2O$⁠, and $kzNO−O$⁠)—each of which is extended to higher temperatures than any previous study and with reduced scatter and uncertainty. Generally, these rate data are consistent with data from the literature, although $τVTNO−Ar$ and $τVTNO−NO$ are observed to differ strongly from both the Millikan and White correlation and Park two-temperature model.