Low-temperature spectroscopy of the 12C2H2 (v1+v3) band in a helium buffer gas

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.