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

TL;DR

This study uses buffer gas cooling with helium to perform low-temperature laser spectroscopy of acetylene, measuring rotational and translational temperatures and analyzing collisional physics relevant to planetary atmospheres.

Contribution

It demonstrates cryogenic spectroscopy of $^{12}$C$_2$H$_2$ with helium buffer gas and estimates elastic collision cross sections at different temperatures.

Findings

Rotational temperatures down to 20 K were achieved.

Elastic collision cross sections were estimated at 25 K and 100 K.

Monte Carlo simulations matched experimental diffusion times.

Abstract

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$^{2}$.