  Cooling molecules in the gas phase is important for precision spectroscopy,
cold molecule physics, and physical chemistry. Measurements of conformational
relaxation cross sections shed important light on potential energy surfaces and
energy flow within a molecule. However, gas-phase conformational cooling has
not been previously observed directly. In this work, we directly observe
conformational dynamics of 1,2-propanediol in cold (6K) collisions with atomic
helium using microwave spectroscopy and buffer-gas cooling. Precise knowledge
and control of the collisional environment in the buffer-gas allows us to
measure the absolute collision cross-section for conformational relaxation.
Several conformers of 1,2-propanediol are investigated and found to have
relaxation cross-sections with He ranging from
$\sigma=4.7(3.0)\times10^{-18}\:\mathrm{cm}^{2}$ to
$\sigma>5\times10^{-16}\:\mathrm{cm}^{2}$. Our method is applicable to a broad
class of molecules and could be used to provide information about the potential
energy surfaces of previously uninvestigated molecules.
