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The reaction of a hydrogen atom with a deuterium molecule can produce an oscillating pattern in the directions its HD products scatter. A 2015 study traced that pattern to quantum interference between different reaction mechanisms that lead to the same outcome. The double-slit experiment is a useful analogy for the interference—not a description of the apparatus or of literal slits in the reaction.
What reaction did the researchers study?
The reaction is H + D2 → D + HD: an incoming hydrogen atom reacts with a deuterium molecule, producing a deuterium atom and a molecule containing one hydrogen and one deuterium atom. The researchers measured state-to-state angular distributions, which describe how products in specified internal energy states emerge at different scattering angles.
In particular, the team reported oscillations in backward scattering for HD products in selected low rotational and vibrational states. The finding is therefore specific to measured product states and scattering directions; it is not a claim that every chemical reaction displays a visible interference pattern.
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How did the experiment reveal the pattern?
Pablo G. Jambrina and co-authors reported the study in Nature Chemistry, volume 7, pages 661–667. It appeared online on 29 June 2015. They used a technique called photoloc to measure the angular distributions of products from H + D2 collisions. The primary paper compares those measurements with rigorous quantum calculations and classical trajectory calculations on an accurate potential energy surface.
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A contemporary account describes preparing cold D2 and HBr in a vacuum chamber, then using a laser pulse to dissociate HBr and initiate reactive collisions. The HD products were analyzed at different angles using state-selective laser ionization and mass spectrometry. These are details of a specialized laboratory experiment, not a procedure for general or home use. Chemistry World’s 2015 report gives this account of the setup.
Why does the angular distribution oscillate?
Different reaction mechanisms can lead to the same product state and scattering direction. In quantum mechanics, the contributions from those alternatives are amplitudes, and amplitudes can interfere. Depending on their relationship, they reinforce or reduce one another, producing peaks and dips in the measured distribution.
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The researchers compared two kinds of calculation to test this interpretation:
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|---|---|---|
| Quasiclassical trajectory calculations | Classical trajectories representing the contributing reaction mechanisms | They include the mechanisms but not their mutual quantum interference, so they do not reproduce the reported oscillatory structure. |
| Rigorous quantum calculations | The quantum dynamics of the reaction, including interference between mechanisms | They reproduce the interference pattern reported in the angular distributions. |
The classical comparison is useful because it distinguishes the existence of multiple reaction routes from the specifically quantum effect of those routes interfering. It does not mean that classical trajectories are useless for describing reaction dynamics.
Is this the chemical equivalent of a double-slit experiment?
Only as an analogy. In a double-slit experiment, alternatives associated with two openings can interfere. Here, the alternatives are distinct molecular reaction mechanisms that produce the same outcome. The experiment did not send atoms through literal slits or use a standard two-slit apparatus; the primary paper describes interference between quasiclassical reaction mechanisms.
Why is the effect not always easy to see?
Chemistry World’s account notes that thermal motion can smear interference, making the pattern harder to observe in many systems. That is context for why a clearly observed pattern is notable, not evidence that interference is absent from other chemical reactions. The result establishes the effect for the particular reaction, product states, and angular distributions studied.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the result means for understanding reactions
The study shows that a reaction’s products can carry information about how the reaction occurred: multiple mechanisms may contribute to the same measured outcome, and their quantum interference can shape the angular distribution. As co-author Richard Zare put it, “simple intuitive concepts will not suffice in general to understand this type of reaction dynamics,” as quoted by Chemistry World. The paper’s significance is not a single headline numerical value; it is the observed oscillatory pattern and its agreement with quantum calculations.
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Sources
- Jambrina et al., “Quantum interference between H + D2 quasiclassical reaction mechanisms,” Nature Chemistry 7, 661–667 (2015), published online 29 June 2015: https://www.nature.com/articles/nchem.2295.
- Philip Ball, “Simple reaction shows quantum interference,” Chemistry World, 1 July 2015: https://www.chemistryworld.com/news/simple-reaction-shows-quantum-interference/8321.article.
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