Polar molecules engaged in pendular states captured by molecular-beam scattering experiments

We demonstrate that when two polar molecules as those of water, ammonia, and hydrogen sulfide encounter each other at a distance much larger than their dimensions they engage a synchronous motion that promotes the transformation of free rotations into coupled pendular states. This discovery has been prompted by high-resolution molecular beam scattering experiments presented here, addressed to the measure of the total integral cross section changes as a consequence of molecular rotation couplings. The experimental observations and the theoretical treatment developed to shed light on the details of the phenomenon suggest that the interplay among free rotations and pendular states depends on the relative velocity, on the rotational levels, and on the dipole moments of the interacting molecules. The features of this intriguing phenomenon may be crucial for the interpretation and the control of basic chemical and biological processes.