Analysis of a non-LTE hypersonic spectrum of methane between 5880 and 6220 cm−1

The SMAUG device is used to thermally populate the vibrational states of the methane molecule and depopulate its rotational states through its supersonic expansion in argon. The methane molecule is probed during expansion by cavity ringdown spectroscopy (CRDS) between 5880 and 6220 cm⁻¹. The laser beam passes through both the isentropic core of the flow, characterized by a very low rotational temperature of 40.4 ± 0.8 K, and the hotter boundary layers surrounding it, which are characterized by an average rotational temperature of 364.3 ± 7.7 K. Analysis of the spectrum reveals that the vibrational population does not follow a Boltzmann distribution. A special procedure is developed to assign an effective vibrational temperature to each vibrational state, ranging from 532.4 ± 32.2 to 1112.0 ± 81.7 K for molecules in the isentropic core, and from 811.6 ± 43.1 to 851.8 ± 45.6 K for those in the shear layer. Only the first vibrational states of each methane polyad remain significantly populated leading to detectable transitions. A total of 2525 transitions from the cold gas of the isentropic core and from the warmer gas of the boundary layers are assigned from a new ab initio effective model and 1000 are confirmed by lower state combination differences (LSCD). Among these transitions, 718 originates from cold bands and 1807 from hot bands starting from vibrational states ν₄, ν₂ (dyad), 2ν₄ (pentad) and 3ν₄ (octad).