Strong thermal nonequilibrium in hypersonic CO and CH4 probed by CRDS

A new experimental setup coupling a High Enthalpy Source (HES) reaching 2000 K to a cw-cavity ring-down spectrometer has been developed to investigate rotationally cold hot bands of polyatomic molecules in the [1.5, 1.7] μm region. The rotational and vibrational molecular degrees of freedom are strongly decoupled in the hypersonic expansion produced by the HES and probed by cavity ring-down spectroscopy. Carbon monoxide has been used as a first test molecule to validate the experimental approach. Its expansion in argon led to rotational and vibrational temperatures of 6.7 ± 0.8 K and 2006 ± 476 K, respectively. The tetradecad polyad of methane (1.67 μm) was investigated under similar conditions leading to rotational and vibrational temperatures of 13 ± 5 K and 750 ± 100 K, respectively. The rotationally cold structure of the spectra reveals many hot bands involving highly excited vibrational states of methane.