High-temperature emission spectroscopy of methane

A high-enthalpy source (HES) has been developed in Rennes either to heat gases up to 2000 K in local thermodynamic equilibrium (LTE) or to generate hypersonic expansions. The HES prototype has been associated with a high-resolution Bruker IFS 120 HR Fourier transform spectrometer to record emission spectra of hot gases, in LTE conditions. A series of emission spectra of methane has been obtained at 1005, 1365, 1485, 1625 and 1820 K in the pentad spectral region located around 3000 cm−1, at Doppler-limited resolution (0.02 cm−1). Spectra have been corrected for the transmission function that strongly affects the infrared radiation emitted by the hot gas. Line-integrated absorption cross sections have been extracted from the corrected spectra using an improved procedure for the calculation of the total partition function Q of methane at high temperature. This calculation included anharmonicity and rovibrational interaction effects, and was based on a multi-resolution fully converged direct partition sum. It has been shown that, as the temperature increases above 1000 K, the commonly used harmonic and rigid rotor double approximation to estimate Q leads to underestimated values.