Dipolar molecules do not stop colliding. In certain low-energy collisions, their effective dipole–dipole interaction weakens, reducing the long-range attraction that can steer them together. The effect has been measured in NO–ND3 collisions and reported in related ammonia collisions; it is a change in how collisions proceed, not an off switch for collisions.
Why does the dipole–dipole interaction weaken?
The key is that the molecules’ internal quantum states matter. In the NO–ND3 experiment, the selected rotational states have opposite-parity partners close in energy. A molecule in a parity eigenstate has no permanent dipole expectation value in zero external field. But the electric field of a nearby collision partner can mix opposite-parity states, giving each molecule an effective dipole. When both molecules polarize one another, their interaction can help draw them into a collision.
This mixing has an energy cost: the separation between the opposite-parity states. The dipole–dipole coupling grows as the molecules get closer, approximately as 1/R3, where R is their separation. At shorter range it can overcome the energy splitting and induce mutual polarization. Farther apart, the coupling is weaker. If it is too weak to mix the states, the effective interaction crosses over toward a 1/R6 dependence instead of retaining the long-range dipolar form.
Lower collision energy changes which part of that interaction is important to the encounter: the molecules move more slowly and the coupling at the relevant larger separations may no longer overcome the parity-state energy cost. This is an energy-dependent crossover, not a universal threshold at which dipoles or collisions disappear.
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What did the NO–ND3 experiment find?
Tang and colleagues measured state-resolved inelastic collisions between NO radicals and ND3 molecules with crossed and merged molecular beams. Their 2023 study covered collision energies from 0.1 to 580 cm−1. At higher energies, correlated rotational excitation reflected electrostatic multipole interactions; at intermediate energies, trajectories could orbit partway around a collision partner, producing a narrow backward-scattering feature.
The analysis used these parity splittings:
| Collision partner | Relevant splitting | Role in the explanation |
|---|---|---|
| NO | 0.0119 cm−1 Λ-doublet splitting, as reported by Tang et al. (2023) | Energy cost for mixing the opposite-parity NO states |
| ND3 | 0.053 cm−1 inversion splitting, as reported by Tang et al. (2023) | Energy cost for mixing the opposite-parity ND3 states |
The interaction competes with the sum of those splittings. The authors report that below 0.2 cm−1, the measured integral cross section departs from the dipolar Langevin-capture trend. They interpreted this as suppressed mutual polarization: “effectively switching off the molecular dipole moments.” That phrase describes the effective interaction in the collision, not a literal cessation of collisions.
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Do the molecules eventually stop scattering?
No. In the NO–ND3 calculations, the integral cross section has a local maximum below about 0.2 cm−1 and then enters the Wigner threshold regime, where it scales as Ecol−1/2. That is a threshold-law change in the energy dependence of the cross section, not a zero cross section.
There is also an important distinction between the lowest-energy measured signal and the inferred field-free behavior. Tang et al. note that, at the lowest energies, up to 50% of detectable events could occur before the beams had fully merged, inside the curved hexapole’s strong, inhomogeneous electric field. Including those field-affected events reconciled the observed slowed increase with their model. The apparatus field therefore matters when interpreting the lowest-energy data; it should not be mistaken for the field-free crossover itself.
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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,” examined state-to-state collisions between ammonia isotopologues from 0.3 to 100 cm−1. It reports a local maximum in cross sections and correlated energy transfer in both collision partners as evidence for suppression of dipole–dipole interactions at low energies. Its scattering calculations explain the scaling in relation to parity-splitting energies.
This is related evidence for parity-mediated suppression, but it is a different molecular system and energy range from the NO–ND3 experiment. The reported 0.3–100 cm−1 range should not be conflated with the below-about-0.2 cm−1 behavior discussed for NO–ND3.
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Is this the same as shielding molecules from collisions?
No. The parity-mixing crossover describes how an interaction changes in field-free molecular-beam collisions; shielding is an engineered way to control encounters or loss. For example, a 2021 KRb experiment used an electric-field-induced shielding resonance and reported a factor-of-30 reduction in reactive loss. A 2024 theoretical study examined static-field shielding barriers and predicted species-dependent effects. A separate 2026 Science abstract reported double-microwave dressing with loss suppression exceeding 10,000 for two-body loss and 1,000 for three-body loss, plus a several-second lifetime.
These shielding results concern different species, setups, observables, and mechanisms. Their reported loss-reduction factors are not directly comparable to a low-energy change in a molecular scattering cross section.
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