We propose a method to study the rotational relaxation of polar molecules [here taking fluoromethane
(CH3F) as an example] in collisions with 3.5 K buffer-gas helium (He) atoms by using an electrostatic guiding technique. The dependence of the guiding signal of
CH3F on the injected He flux and the dependence of the guiding efficiency of
CH3F on its rotational temperature are investigated both theoretically and experimentally. By comparing the experimental and simulated results, we find that the translational and rotational temperatures of the buffer-gas cooled
CH3F molecules can reach to about 5.48 and 0.60 K, respectively, and the ratio between the translational and average rotational collisional cross sections of ${\mathrm{CH}}_{3}\mathrm{F}\ensuremath{-}\mathrm{He}$ is $\ensuremath{\gamma}={\ensuremath{\sigma}}_{t}/{\ensuremath{\sigma}}_{r}=36.49\ifmmode\pm\else\textpm\fi{}6.15$. In addition, the slowing, cooling, and boosting effects of the molecular beam with different injected He fluxes are also observed and their forming conditions are investigated in some detail. Our study shows that our proposed method can not only be used to measure the translational and rotational temperatures of the buffer-gas cooled molecules, but also to measure the ratio of the translational collisional cross section to the average rotational collisional cross section, and even to measure the average rotational collisional cross section when the translational collisional cross section is measured by fitting the lifetime of molecule signal to get a numerical solution from the diffusion equation of buffer-gas He atoms in the cell.
Buffer gas cooling, one of the most relevant direct cooling techniques for cooling molecules, relies on dissipating the energy of the molecule via collisions with a buffer gas. The cooling efficiency hinges on the molecule-atom scattering properties, concretely, on the transport properties. This work presents a global study on the interactions, collision dynamics, and transport properties of monofluoride molecules (X-F), with X being a metal, in the presence of a cold He buffer gas. The interactions are calculated using ab initio quantum chemistry methods, and the dynamics is treated fully quantal, assuming the monofluoride molecule is a rigid rotor. The resulting thermalization and rotational quenching rates are analyzed in light of the distorted-wave Born approximation, yielding an explanation based on the elemental physical properties of the molecule under consideration. Therefore, our findings contribute to understanding the rotational quenching of molecules in a cold buffer gas.