Spectroscopic modeling and measurements of the CN Violet and Red systems for the development of nonequilibrium temperature and speciation diagnostics

Abstract

The uniquely strong radiative properties of the cyano-radical (CN) make it an important species to model in high-enthalpy, nonequilibrium flows containing carbon and nitrogen. In this work, spectroscopic models for the Violet and Red systems of CN have been developed and validated at wavelengths in both the ultraviolet (UV) and near-infrared (NIR) regimes that would provide quantitative CN rovibrational temperature and concentration. The ultraviolet wavelengths chosen probe high-lying (J′′=30.5−40.5) and low-lying (J′′=0.5−5.5) rotational states in the ground vibrational level (v′′=0) of CN along with high-lying rotational states in the third excited vibrational level (v′′=3) of CN. Meanwhile, the NIR wavelengths cover a range of lower rovibrational content, including high-lying and low-lying rotational levels from the ground up to the fourth excited vibrational level of CN. Dilute cyanogen (C2N2) in argon (Ar) mixtures were shock heated to produce stable quantities of CN that could be spectroscopically studied. In the UV, fixed-temperature absorption cross-section measurements at different wavelengths and a fixed-wavelength temperature sweep for the chosen diagnostic wavelengths enabled an evaluation of the different spectroscopic models and their ability to correctly infer the post-shock temperature in mixtures with high Ar dilution. In the NIR, the lasers were scanned at 100 kHz to probe multiple transitions over different rovibrational states over a wide range of temperatures, and both Einstein A’s and relative line positions of observed Red transitions of CN were experimentally quantified.