Low-temperature spectroscopy of the (C2H2)-C-12 (upsilon(1) + upsilon(3)) band in a helium buffer gas

Buffer gas cooling with a He-4 gas is used to perform laser-absorption spectroscopy of the (C2H2)-C-12 (upsilon(1) + upsilon(3)) 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 (C2H2)-C-12 in the buffer cell is measured against the He-4 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: sigma(el)(100 K) = (4 +/- 1) x 10(-20) m(2) and sigma(el)(25 K) = (7 +/- 2) x 10(-20) m(2).