Physicists in the Netherlands have demonstrated that the collision rate between polar molecules drops sharply as their energy approaches zero, a counterintuitive result that contradicts the classical picture of how electric dipoles interact. The finding, published in Nature Chemistry, provides the first experimental confirmation of a quantum mechanical effect that had previously been studied only in theory.
At the heart of the discovery is the concept of the electric dipole moment, the asymmetric charge distribution that makes molecules such as ammonia (NH3) polar. In the classical view, these molecules should attract each other more strongly as they slow down, leading to a continuously increasing collision cross-section. But experiments by Bas van de Meerakker and his colleagues at Radboud University Nijmegen show the opposite: as energy decreases, the cross-section falls off dramatically.
The explanation lies in quantum mechanics. An isolated molecule in a specific quantum state has a definite parity, meaning its net dipole moment in the laboratory frame is exactly zero. When two such molecules collide at high speeds, their electrostatic fields mix these opposite parity states, effectively switching on each other's dipole moments and allowing the molecules to attract. At low speeds, however, the molecules cannot get close enough to trigger this mixing. Their dipole moments remain off, and collisions become far less likely.
Observing this effect required overcoming a significant experimental hurdle. Merging two beams of cold molecules normally relies on electric fields to steer them, but when the molecules have similar dipole moments, steering one beam inevitably disturbs the other. The team solved this by combining a 2.6-meter Stark decelerator, a curved hexapole guide, and a merged quadrupole/hexapole trap into a single apparatus that overlaps the beams at precisely the right position with minimal angle between them.
With this setup, the researchers verified the predicted energy-dependent changes in cross-section for NH3-NH3 collisions. They also showed that the dependence changes when hydrogen atoms are replaced with deuterium, as in NH3-ND3 or ND3-ND3 collisions, confirming the quantum mechanical origin of the effect.
The results have practical implications for ultracold chemistry. According to van de Meerakker, the drop in cross-sections at low energies could make certain scattering experiments more difficult, particularly those using crossed or merged beams. But trap-based experiments stand to benefit, because lower inelastic cross-sections mean fewer molecules are lost from the trap, extending the time available for study.
Looking ahead, the team plans to introduce a controlled electric field into the beam overlap region. The mechanism they discovered is expected to respond extremely sensitively to such a field, with even small changes altering cross-sections by orders of magnitude. This would provide a tunable knob for controlling collision outcomes, a prospect van de Meerakker describes as exciting and one that originally motivated his interest in cold molecules two decades ago.
Tim Langen, a physicist at the Vienna University of Technology who was not involved in the study, called the work important because it provides experimental evidence for a phenomenon that had mainly been discussed theoretically. He described the experimental approach as impressive for enabling measurements in a previously inaccessible regime, and said he expects the findings to be of broad interest to researchers working on cold and ultracold molecules.
The discovery could also inform efforts in quantum simulation and computation, where precise control over molecular interactions is essential. By understanding when and how dipole moments switch off, scientists may gain a new tool for manipulating the building blocks of future quantum technologies.
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