Broadband ultrafast-laser-absorption measurements of NO2 absorption cross-sections from 430 to 500 nm at high temperatures behind shock waves

This work presents broadband spectrally resolved measurements of the absorption cross-section of NO2 from approximately 430 to 500 or 580 nm, depending on temperature, with a spectral resolution of approximately 0.06 nm. An ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic was developed and used to provide broadband (up to approximately 900 cm−1 per laser shot) cross-section measurements with sub-nanosecond time resolution. Cross-section measurements were acquired from 430 to 580 nm in a heated gas cell at temperatures of 296 K and 670 K and a pressure of 1.5 bar. Cross-section measurements were also acquired behind reflected shock waves at temperatures near 970 K, 1150 K, and 1450 K at pressures near 1.5 bar. A quantum-cascade-laser-absorption diagnostic for NO was used to measure NO mole fraction at 500 kHz and determine the extent of NO2 decomposition in shock-tube experiments. The measured cross-sections typically exhibit good agreement with prior measurements available at select wavelengths and conditions as well as with recently developed theoretical predictions at most wavelengths and temperatures studied. A detailed description of the experimental and data processing procedures that enabled the ULAS measurements despite pronounced variations in laser pulse energy and the absence of non-resonant wavelengths within the pulse bandwidth are also presented.