Low-temperature spectroscopy of the \(^{12}\)C\(_2\)H\(_2\) (\(\upsilon_1 +\upsilon_3\)) band in a helium buffer gas

Buffer gas cooling with a \(^4\)He gas is used to perform laser-absorption spectroscopy of the \(^{12}\)C\(_2\)H\(_2\) (\(\nu_1+\nu_3\)) 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 underlying helium-acetylene collisional physics, relevant for modeling planetary atmospheres, is also addressed. In particular, the diffusion time of \(^{12}\)C\(_2\)H\(_2\) in the buffer cell is measured against the \(^4\)He 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: \(\sigma_{el}(100\ {\rm K})=(4\pm1)\cdot 10^{-20}\) m\(^{2}\) and \(\sigma_{el}(25\ {\rm K})=(7\pm2)\cdot 10^{-20}\) m\(^{2}\).