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Abstract

Buffer gas cooling with a ⁴He gas is used to perform laser-absorption spectroscopy of the ¹²C₂H₂ (υ₁ + υ₃) band at cryogenic temperatures. Doppler thermometry is first carried out to extract translational temperatures from the recorded spectra. Then, rotational temperatures down to 20 K are retrieved by fitting the Boltzmann distribution to the relative intensities of several ro-vibrational lines. The potential of our setup to tune the thermal equilibrium between translational and rotational degrees of freedom is also demonstrated. This can be used to reproduce in a controlled way the regime of non-local thermal equilibrium typically encountered in the interstellar medium. The underlying helium-acetylene collisional physics, relevant for modeling planetary atmospheres, is also addressed. In particular, the diffusion time of ¹²C₂H₂ in the buffer cell is measured against the ⁴He flux at two separate translational temperatures; the observed behavior is then compared with that predicted by a Monte Carlo simulation, thus providing an estimate for the respective total elastic cross sections: σel(100 K) = (4 ± 1) × 10^-20 m² and σel(25 K) = (7 ± 2) × 10^-20 m².