Line mixing study on the fundamental rovibrational band of nitric oxide near 5.3 μm

•Measured nitric oxide absorption from 1700 to 2000 cm−1 at 293–802 K, 20–34 atm.•Revealed breakdown of a superposition of Lorentzian profiles due to line mixing.•Constructed a Modified Exponential Gap inter-branch line mixing model.•Explored an alternative modeling approach in the absence of broadening parameters.•Found reasonable agreement between an Energy Corrected Sudden model and data. In this work, we report quantitative absorbance measurements of nitric oxide (NO) diluted in nitrogen between 1700 to 2000 cm−1 and present three line mixing modeling approaches for the measured spectra. Static cell measurements were taken using a narrow-linewidth, external-cavity quantum-cascade laser at temperatures of 293 K and 802 K and pressures of 20–34 atm. The measured results exhibit considerable deviations from the spectra simulated by a superposition of Lorentzian line profiles due to significant line mixing coupling effects at high-number-density conditions. Our previous work demonstrated a line mixing model based on relaxation matrix theory and the Modified Exponential Gap (MEG) law for the NO R-branch. With expanded access to the P- and Q-branches, the measured data indicated significant line mixing effects between lines of different branches in addition to those within the same branch. An empirical two-scaling-factor inter-branch MEG model is presented that delivers strong agreement across the measured spectra, with residuals less than 2% for the spectrum at 293 K and 34 atm. In addition, the Energy Corrected Sudden (ECS) scaling law is shown to produce reasonable agreement across the measured spectra, excluding the Q-branch. In the Q-branch peak, the ECS model overpredicts the measured data by about 7%. The different line mixing models presented and discussed in this work will improve NO absorption predictions vital for laser absorption applications in high-number-density gas conditions. Future studies may seek to account for inter-spin-split coupling to further improve the ECS application to NO absorption.