The classical expectation is simple and seductive. In the Langevin capture model, long the workhorse for predicting how strongly interacting molecules scatter off one another, the collision cross-section grows as the collision energy to the power of minus two-thirds. Cool the gas down, and molecules should collide more and more readily, because the long-range dipole–dipole attraction reaches out ever further relative to the particles’ diminishing kinetic energy. Generations of researchers have used this scaling to design cold-molecule experiments and to estimate whether trapped ensembles of polar molecules can be evaporatively cooled. The Radboud measurements, published in Nature Chemistry, show that this common wisdom collapses precisely where it matters most: at the lowest collision energies.

The reason lies in a quantum subtlety with no classical analogue. A molecule in a definite parity state, meaning a wavefunction that is either even or odd under inversion of all coordinates, has exactly zero dipole moment in the laboratory frame. A dipole only emerges when wavefunctions of opposite parity are mixed, for example by an external electric field or by interactions with the environment. Many chemically important molecules, including ammonia, hydroxyl radicals, nitric oxide and formaldehyde, possess rotational levels split into two nearly degenerate components of opposite parity, so-called parity doublets. When two such molecules approach each other, the dipole–dipole interaction itself couples the opposite-parity states, polarizing each partner and switching on effective dipole moments. This mutual self-polarization restores the Langevin behaviour at high energies.

But as the collision energy drops, the molecules no longer penetrate close enough to sustain that polarization. The classical turning point of the encounter moves to larger intermolecular distances, where the dipole–dipole interaction weakens from its characteristic inverse-cube dependence toward an inverse-sixth form. Once the interaction energy falls below the parity doublet splitting, the effective dipole moments effectively switch off, and the cross-section plummets, in some cases by orders of magnitude. Theory had predicted that this collapse produces a characteristic local maximum in the scattering cross-section, with the cross-section rising again only at still lower energies as dictated by the Wigner threshold law. Until now, that maximum had never been observed.

Reaching the required energies posed a formidable technical challenge. The local maximum is predicted to occur at collision energies tied to the parity splitting, which for most molecules is far below one wavenumber, equivalent to a temperature of roughly 1.4 kelvin. Earlier merged-beam experiments on nitric oxide colliding with deuterated ammonia reached a minimum energy of 0.15 wavenumbers, still too high. Worse, the standard beam-merging technique fundamentally fails for two molecules with similar dipole moments and masses, because bending one beam to merge it with the other deflects the second in the opposite direction, destroying the overlap needed for collisions to occur. That limitation had ruled out the most natural candidate systems, including combinations of ammonia and formaldehyde.

The Nijmegen group circumvented the problem with an advanced beam-merging protocol that combines a 2.6-metre Stark decelerator, a curved hexapole guide and a merged guide in which two adjacent quadrupole traps gradually evolve into a single hexapole trap. The two packets emerge nearly co-propagating, colliding at an effective crossing angle of just two degrees in a near-field-free region, which yields collision energies as low as about 0.3 wavenumbers. Using isotopically labelled nitrogen atoms to spectroscopically separate the signals from the two beamlines, and a recoil-free vacuum-ultraviolet ionization scheme to sharpen the detection, the team measured fully state-resolved integral cross-sections for deuterated and normal ammonia colliding with themselves and with each other, spanning energies from 0.3 to 100 wavenumbers.

The results were unambiguous. For every ammonia isotopologue combination, the cross-section followed the Langevin inverse two-thirds power law at high energies, then turned over into a clear local maximum whose position shifted to higher energies as the parity splitting increased. For the mixed deuterated and normal ammonia system, the energy window was wide enough to reveal a plateau on the low-energy side of the maximum, while for normal ammonia pairs the scattering signal dropped below detectable levels below four wavenumbers. Quantum coupled-channels calculations on an ab initio ammonia dimer potential energy surface reproduced the measured curves in good agreement, validating both the existence of the local maximum and its dependence on the parity splitting.

Beyond confirming the prediction, the team uncovered a deeper universality. By recasting the cross-sections in terms of a characteristic dipolar length and the corresponding dipolar energy, they showed that the peak position obeys the power law in which the scaled peak energy scales as the scaled parity splitting to the four-thirds power. The maximum therefore depends not only nonlinearly on the splitting but also on the masses and dipole moments of the colliding molecules, a significant revision of the earlier linear scaling derived from a purely classical capture model. Complementary measurements on nitric oxide colliding with ammonia demonstrated that the phenomenon is not an ammonia quirk but a generic feature of any collision system built from parity-doublet molecules.

The most direct evidence for the switching-off mechanism came from velocity map imaging, which recorded the correlated energy transfer between both collision partners. When the dipole–dipole interaction dominates, both molecules flip their parity during the collision, and the energy released by the undetected partner imparts a measurable recoil to the detected one, enlarging the scattering image. When the weaker dipole–quadrupole interaction takes over, only the detected molecule flips parity and the partner scatters elastically, leaving the image tiny. At the lowest energies probed, the deuterated ammonia images collapsed to a structureless dot, proving that the partner molecule scattered elastically and that the dipole–dipole interaction had indeed been suppressed. For nitric oxide with ammonia, a central dot surrounded by a halo revealed both interactions contributing simultaneously, a bimodal regime that theory reproduced in striking detail.

The findings cut both ways for the cold-molecule community. On one hand, the suppressed inelastic cross-sections below the local maximum could hamper evaporative cooling of trapped polar molecules and make detailed scattering experiments harder than anticipated, since trap experiments typically operate right in the energy regime of the maximum. On the other hand, the effect opens a powerful control knob: because opposite-parity states in molecules such as ammonia and hydroxyl mix already at electric fields of one kilovolt per centimetre or less, modest external fields should drastically reshape the cross-sections, and time-varying fields could toggle collisions between a dipole–dipole-dominated and a dipole–quadrupole-dominated regime at will. For molecules that also carry a magnetic dipole moment, magnetic fields offer a further handle on the parity splitting itself. What began as a search for a theoretical curiosity has ended with a counterintuitive new rule for the coldest chemistry, and a roadmap for controlling it.

Subject of Research: Experimental observation of dipole–dipole interaction dynamics and a local cross-section maximum in cold collisions between ammonia isotopologues

Article Title: Evolution of dipole–dipole dynamics in cold ammonia collisions

Article References: Chang, Y., van Roij, A. J. A., Kuijpers, S., Herbers, S., Walraven, E. F., Karman, T., & van de Meerakker, S. Y. T. (2026). Evolution of dipole–dipole dynamics in cold ammonia collisions. Nature Chemistry. https://doi.org/10.1038/s41557-026-02270-y

Image Credits: AI Generated

DOI: 10.1038/s41557-026-02270-y

Keywords: cold molecules, dipole–dipole interaction, ammonia, parity doublet, molecular collisions, Stark deceleration, velocity map imaging, Langevin capture model, ultracold chemistry, cross-sections, quantum scattering, external field control