Development and demonstration of a two-color nitric oxide vibrational temperature diagnostic using spectrally-resolved ultraviolet laser absorption

Abstract

Development of a new ultraviolet (UV) laser absorption diagnostic has enabled the probing of nitric oxide (NO) in the second excited vibrational state (v” = 2) for inferences of quantum-state-specific number density and vibrational temperature time-histories. Spectroscopic modeling informed the selection of the new 246.3222 nm wavelength, as this wavelength exhibits high sensitivity for thermometry in the 2000 – 8000 K temperature range. This 246.3222 nm absorption feature consists of contributions from the $R_{12}(24.5)$, $R_{11}(15.5)$, $Q_{22}(24.5)$, and $Q_{21}(15.5)$ transitions all originating in the v” = 2 state. Absorption cross-sections at this selected wavelength were measured in reflected shock experiments for sweeps of both wavelength and temperature. The wavelength sweep investigated cross-sections over a 246.3202 – 246.3246 nm range at 4590 K, and the temperature sweep measured cross-sections over a 2500 – 7500 K range at the peak of the absorption feature (246.3222 nm). Cross-section results agree with the Stanford NO gamma-band model to within $\pm$5%, confirming the use of the model for subsequent thermometry studies. Thermometry was demonstrated in reflected shock experiments probing the vibrational relaxation and chemical reactions in 2% NO diluted in either argon (Ar) or nitrogen (N${}_2$). These experiments leverage previous UV laser absorption diagnostics that probe NO in the ground vibrational state (v” = 0) using the R₁₁(26.5), R₁₂(34.5), Q₂₁(26.5) , and Q₂₂(34.5) transitions near 224.8155 nm and the Q₁₁(12.5) , R₁₂(19.5) , P₂₁(12.5) , and Q₂₂(19.5) transitions near 226.1026 nm, which were studied in Ref. [1]. The combination of the new diagnostic wavelength with previously validated diagnostics yields low-uncertainty vibrational temperature time-histories that are in excellent agreement with previously inferred vibrational relaxation time results from Refs. [2] and [3]. Future work will apply this two-color nitric oxide vibrational temperature diagnostic to probe the vibrational temperature of NO formed in high-temperature, shock-heated air at conditions relevant to hypersonic and reentry vehicles.