Effects of pressure variations on electronic-resonance-enhanced coherent anti-Stokes Raman scattering of nitric oxide

The effects of pressure variations on the electronic-resonance-enhanced coherent anti-Stokes Raman scattering (ERE-CARS) signal of nitric oxide (NO) were studied at pressures ranging from 0.1 to 8 bar. ERE-CARS signals were recorded in a gas cell filled with a mixture of 300 ppm NO in N2 buffer gas at room temperature. The ERE-CARS signal was found to increase with rising pressure up to 2 bar and to remain nearly constant thereafter. The spectra recorded at different cell pressures were modeled using a modified version of the Sandia CARSFT code. Laser-saturation effects were accounted for by systematically varying the theoretical ultraviolet probe-laser linewidth. Excellent agreement was obtained between theory and experiment for the pressure-scaling behavior of the ERE-CARS signal of NO. This finding, along with a negligible influence of electronic quenching on the ERE-CARS signal, provides strong incentive for the application of ERE-CARS to measurements of NO concentrations in high-pressure combustion environments.