Photoionization-dominated, two-photon laser-induced fluorescence measurements of CO in plasma-induced carbon ablation

A technique for quantitative two-photon laser-induced fluorescence measurements of the COB¹Σ⁺←X¹Σ⁺(v^''=0) Hopfield-Birge system in high-enthalpy environments is shown. The two-photon transition is pumped by high-intensity, nanosecond laser excitation to produce photoionization rates much greater than the quenching rates at the probe volume, which simplifies the quantification of the signal. We demonstrate this technique along the stagnation streamlines of a graphite ablator exposed to the 6000 K plume of an atmospheric-pressure inductively coupled air plasma torch. Additionally, the collected LIF signal in the boundary layer was calibrated to absolute number density using the plasma torch plume as a reference condition and assuming thermodynamic equilibrium. The results show CO number densities as high as 9×10²³m^-3 approximately 100 µm from the graphite surface and a monotonically decreasing number density as the LIF probe volume is moved away from the surface, indicating diffusion of CO from the surface into the impinging jet. The diffusion length scale of CO at these conditions, which is defined here as the distance into the flow where the CO number density reaches 5% of its maximum value, is approximately 2 mm. In a subsequent experiment, a second rotational line of the COB¹Σ⁺←X¹Σ⁺(v^''=0) Hopfield-Birge system was pumped to obtain a two-line rotational temperature. Using this technique, we measured temperatures in the boundary layer from 3000 K near the sample to about 6000 K towards the edge of the CO layer. These temperature measurements agree well with prior measurements using coherent anti-Stokes Raman scattering along the stagnation streamline of the graphite ablator. This technique presents a potential road map for quantitative two-photon LIF of CO in high-temperature, complex reacting flows where the quenching environment of the target species is uncertain. We present an analysis of the photophysics in the photoionization-dominated regime, as well as discuss the effects of spectral and photolytic interferences on the LIF signal.