Characterization of Non-Equilibrium CN via the B2Σ+ ← X2Σ+ and A2Π ← X2Σ+ Electronic Systems Using Laser Absorption Spectroscopy

This article presents laser absorption spectroscopy (LAS) measurements of CN internal temperatures and mole fraction acquired behind reflected shock waves in a mixture of 2.2% CH4 and 0.1% H-2 in N-2 at conditions relevant to entry into Titan's atmosphere. Both Ultrafast Laser Absorption Spectroscopy (ULAS) and Tunable Diode Laser Absorption Spectroscopy (TDLAS) were employed to compare measurements acquired via the B-2 Sigma(+) <- X-2 Sigma(+) (violet) and A(2)Pi <- X-2 Sigma(+) (red) systems, respectively. Measurements were acquired at a test condition where the pressure behind the reflected shock wave was 0.15 bar and the vibrationally and chemically frozen temperature was 5360 K. The broadband ULAS measurements were well modeled by a two-temperature absorption spectroscopy model which indicates that the rotational and vibrational state populations of CN each follow or nearly follow a Boltzmann distribution at a unique temperature. The ULAS and TDLAS rotational temperature measurements agreed well with each other while the mole fraction of CN measured by TDLAS was approximately two times larger. This discrepancy in the measured mole fraction of CN is approximately two times smaller than observed previously at higher pressures and comparable temperatures where pronounced non-Boltzmann state populations were observed via B-2 Sigma(+) <- X-2 Sigma(+) measurements. Together these results suggest that non-equilibrium state populations in the upper and, perhaps lower, electronic states are at least partially responsible for the apparent differences in ULAS and TDLAS measurements although further research is needed to test this hypothesis.