Hypersonic Methane Probed by CRDS

A new cavity ring-down spectroscopy device, designed to study hypersonic gas jets, started to yield encouraging results at the Institut de Physique de Rennes (IPR). In this experiment, two mixtures have been used: first, a mixture of argon and carbon monoxide and second, a mixture of argon and methane. Mixtures were contained at high pressure in a reservoir (∼ 1000 Torr) and heated at very high temperature (∼ 2000 K), thanks to the high enthalpy source developed at IPR¹. A hypersonic gas expansion was produced in a low-pressure chamber (∼ 0.09 Torr) connected to the reservoir. High-resolution spectra of methane have thus been investigated in the [59206030] cm⁻¹ spectral range, which corresponds to the Tetradecad region of methane. The CRD spectrometer, developed by the LAME group from the LIPhy laboratory in Grenoble, has been placed perpendicularly to the axis of the jet to record these spectra.

The precise analysis of the CO lines helped to determine the different parameters of the jet: translational, rotational and vibrational temperatures, concentrations of the hot and cold gas. One of the most noteworthy results is the evidence of a strong thermodynamic disequilibrium of CO, with a rotational temperature of 7 K and a vibrational temperature of 2000 K. This confirms the weak vibrational relaxation of the molecules in the jet. Then, the parameters determined for CO gave a starting point for the simulation of the spectrum of methane. Thereupon, they were fitted in order to make the simulation match the experimental results for CH₄. The simulation for CH₄ was obtained from the HITRAN 2012 linelist², for the cold bands (Tetradecad-GS), and from a variational calculation from the GSMA laboratory in Reims³, for the hot bands (Icosad-Dyad). The vibrational temperature is finally about 750 K and the rotational temperature, about 13 K. This rotational simplification is particularly useful for the identification of hot band lines, that is very more difficult in a spectrum at thermal equilibrium.