Low-temperature spectroscopy of the ${}^{12}$C${}_2$H${}_2$ (${\upsilon }_1+{\upsilon }_3$) band in a helium buffer gas

Abstract

Buffer gas cooling with a ${}^4$He gas is used to perform laser-absorption spectroscopy of the ${}^{12}$C${}_2$H${}_2$ ($\hat{I}{½}_1+\hat{I}{½}_3$) 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: $\ddot{I}{ƒ}_{el}(100\mathrm{K})=(4\pm 1)\cdot 10^{-20}$ m${}^2$ and $\ddot{I}{ƒ}_{el}(25\mathrm{K})=(7\pm 2)\cdot 10^{-20}$ m${}^2$.