Vibrational-state-resolved relaxation and chemistry of carbon monoxide and nitrogen mixtures from 2000–10 250 K probing carbon monoxide in the ground to twelfth excited vibrational levels

Vibrational relaxation of the ground to twelfth excited vibrational levels (v″ = 0–12) of carbon monoxide (CO) has been probed with multi-pass absorption spectroscopy (MPAS) on a ring amplified shock tube (RAST) targeting absorption transitions near 5055 nm (1978 cm−1). Streicher et al., “High-temperature measurements of nitrogen vibrational relaxation through pathlengthamplified probing of carbon monoxide vibrational states in shock-tube experiments,” AIAA Paper No. 2025–1991, 2025. Experiments probed mixtures of 0.02%–5% CO diluted in nitrogen (N2) from 2000 to 7500 K and 0.07 to 0.84 atm, with conditions selected for sensitive inference of N2–N2 vibration-translation (VT) relaxation times (τVTN2−N2) using CO as an optically accessible tracer of N2. Two additional types of experiments leveraged argon (Ar) dilution of either 50% or 91%; 50% Ar was used to extend the test time, while 91% Ar was used to access a condition at 10 250 K where chemical reactions occur. High-temperature experiments probed 24 absorption features covering all vibrational states v″ = 0–12 and many rotational levels from J″ = 2–116, as well as weak features from the C13O16 isotopologue. Low-lying vibrational states follow an expected Boltzmann distribution, although states above v″ = 5 are observed to populate faster than expected from their Boltzmann populations—both for full N2 dilution and for 50% Ar dilution. Modal temperatures, inferred from v″ = 0–5 states, remain in good agreement with simulations of the vibrational relaxation processes based on literature values of vibrational relaxation times. Non-Boltzmann behavior for v″ > 5 was investigated with a state-to-state model, with model predictions improving when multiquantum transitions were included in the model. Overall, the inferred values of τVTN2−N2 remain consistent with literature values, although the low scatter and uncertainty of the RAST measurements suggest a rate approximately 10% faster than the Millikan and White correlation.