Dipolar molecules do not stop colliding at low energies. In some systems, their effective long-range dipole–dipole interaction weakens as the molecules approach one another more slowly. That change can reduce collision capture compared with a simple dipolar model, but it does not make the collision cross section vanish.
What changes when the collision energy falls?
In the NO–ND3 experiment reported by Tang and colleagues in 2023, the molecules’ selected rotational states were parity eigenstates. In zero external electric field, those states have no permanent dipole expectation value. Each molecule nevertheless has a nearby state of opposite parity: the NO Λ-doublet partner and the ND3 inversion partner.
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Dipole–dipole coupling can mix these opposite-parity states. When the coupling is strong enough, it produces effective dipoles on both molecules, which then mutually polarize one another. The interaction has a long-range 1/R3 dependence, where R is the distance between the molecules.
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Why does weaker coupling change the interaction?
As the molecules get farther apart, their dipole–dipole coupling weakens. If it becomes too weak to overcome the relevant parity splittings, it can no longer mix the opposite-parity states as effectively. The induced dipoles and their mutual polarization are suppressed.
In the NO–ND3 interpretation, the effective interaction then crosses away from the long-range 1/R3 dipolar form toward a 1/R6 dependence. The molecules are still interacting and can still collide; rather, the strong dipolar contribution that helps draw them into a collision no longer applies in the same way at those larger separations.
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What the experiments found—and what “low energy” means here
Tang and colleagues measured state-resolved inelastic collisions between NO radicals and ND3 molecules over 0.1–580 cm−1, using crossed and merged molecular beams. At higher energies, rotational excitation was correlated with electrostatic multipole interactions. At intermediate energies, the scattering showed a narrow backward feature associated with trajectories that orbit partway around a collision partner. At the lowest energies, the integral cross section departed from the dipolar Langevin-capture trend.
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The authors reported that below about 0.2 cm−1, the Langevin capture model broke down, interpreting the result as suppressed mutual polarization that effectively switches off the molecules’ dipole moments. “Switches off” describes the effective dipoles in this collision regime, not the disappearance of the molecules, the collision itself, or all molecular interactions.
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Do collisions disappear at the threshold?
No. In the modeled NO–ND3 integral cross section, the low-energy behavior includes a local maximum below about 0.2 cm−1, followed by entry into the Wigner threshold regime. There, the modeled cross section scales as Ecol−1/2, where Ecol is the collision energy. That is a threshold-law change in how the cross section depends on energy—not a prediction that the cross section becomes zero.
The measured lowest-energy signal also needs a qualification. The 2023 paper notes that some collisions took place before the beams had fully merged, inside the curved hexapole’s strong, inhomogeneous electric field. At the lowest energies, the authors estimated that up to 50% of detectable events could have occurred there. Accounting for those field-affected events reconciled the observed slowed increase with the modeled low-energy behavior. The experimental signal under those conditions should not be treated as identical to a field-free collision curve.
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How does the ammonia result relate?
A separate 2026 Nature Chemistry report, “Evolution of dipole–dipole dynamics in cold ammonia collisions,” describes state-to-state collisions between ammonia isotopologues over 0.3–100 cm−1. It reports a local maximum in cross sections and correlated energy transfer in both collision partners as evidence of low-energy suppression of dipole–dipole interaction. Its calculations relate the scaling to parity-splitting energies.
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How is this different from molecular shielding?
Low-energy suppression from parity-state mixing is not the same as engineered shielding in trapped ultracold gases. In the former, the effective dipole interaction weakens when coupling cannot overcome internal-state splittings. Shielding instead uses external control to create a barrier that reduces close approach or loss.
- A 2021 KRb experiment reported an electric-field-induced shielding resonance that reduced reactive loss by a factor of 30 and enabled direct thermalization and dipolar evaporative cooling.
- A 2024 theoretical study examined static-field shielding barriers and predicted that their effects vary by molecular species, including differences in scattering length and bound-state behavior.
- A 2026 Science abstract reported a distinct double-microwave-dressing method, with reported suppression exceeding 10,000 for two-body loss and 1,000 for three-body loss, plus a several-second lifetime.
These are separate control approaches and measurements, so their reported suppression factors are not directly comparable with the change in a molecular-beam collision cross section.
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