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 { K } ) = ( 4 \pm 1 ) \cdot 10 ^ { -20 } m ^ { 2 } and \sigma _ { el } ( 25 { K } ) = ( 7 \pm 2 ) \cdot 10 ^ { -20 } m ^ { 2 } .