Researchers in the Netherlands have experimentally shown that molecules which are normally strongly polar can have their dipole moment effectively 'switch off' during low-energy collisions — a result that validates the quantum-mechanical picture of polarity and could give physicists a new knob for controlling chemistry near absolute zero.
The experiment, by Bas van de Meerakker's group at Radboud University Nijmegen and colleagues, is published in Nature Chemistry. It follows theoretical work from 2023 in which the team's simulations predicted that the collision cross-section between ammonia (NH3) molecules should fall off sharply as energy decreases — contradicting the classical picture, in which a polar molecule's lopsided charge distribution should make collisions more likely at lower speeds.
The quantum explanation is subtle. A polar molecule isolated in a specific quantum state has a definite parity, meaning its net dipole moment measured in the lab frame is exactly zero. At high collision energies, the electrostatic fields of two approaching molecules mix opposite-parity states and 'turn on' each other's dipoles, boosting the collision rate. At low energies the molecules never get close enough to trigger that mixing, so the dipoles stay off and the interaction drops away.
Proving this required clearing a hard experimental hurdle. Merging two beams of cold molecules normally uses electric fields — and when the molecules in both beams have nearly identical dipole moments, steering one beam also deflects the other, so the beams may never overlap. The team combined a 2.6-metre Stark decelerator, a curved hexapole guide and a merged quadrupole/hexapole trap to overlap the beams at just the right position and with a minimal angle between them.
They then confirmed the predicted energy dependence for NH3–NH3 collisions, and showed that it changes when hydrogen is replaced with deuterium (NH3–ND3 and ND3–ND3 collisions). Tim Langen, a physicist at TU Wien who was not involved in the work, called the experimental approach impressive and the result convincing, and said he expects it to be of broad interest to researchers working on cold and ultracold molecules.
The practical consequences cut both ways. Lower inelastic cross-sections make merged-beam scattering experiments harder, because scattering becomes harder to observe; but they also mean fewer molecules are lost from traps. The team's next step is to add a controlled electric field to the collision region. The mechanism is expected to respond extremely sensitively, with even small fields potentially changing cross-sections by orders of magnitude.
