High-speed mid-infrared laser absorption spectroscopy of CO$$_2$$ for shock-induced thermal non-equilibrium studies of planetary entry

Abstract

AbstractA high-speed laser absorption technique is employed to resolve spectral transitions of CO$$_2$$ 2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO$$_2$$ 2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO$$_2$$ 2 asymmetric stretch fundamental bands ($$\Delta $$ Δ v$$_3$$ 3 = 1) near 4.19 $$\upmu \hbox {m}$$ μ m . The sensor probes a wide range of rotational energies as well as two vibrational states (00$$^0$$ 0 0 and 01$$^1$$ 1 0). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO$$_2$$ 2 non-equilibrium with a single vibrational temperature.