Imaging resonances in low-energy NO-He inelastic collisions

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Science  13 Nov 2015:
Vol. 350, Issue 6262, pp. 787-790
DOI: 10.1126/science.aad2356

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Watching collisions in the slow lane

Quantum mechanics aims to “micromanage” the details of collisions between atoms and molecules. However, it's hard to discern all the subtleties under high-energy conditions. Vogels et al. slowed down two intersecting beams of helium atoms and nitric oxide (NO) molecules to a relative crawl in order to characterize the collisions precisely. The data revealed short-lived resonances that matched theoretical predictions remarkably well—a striking feat on both sides, given the challenge of accurately modeling NO's unpaired electron. The study highlights chemists' increasingly sophisticated understanding of collision dynamics.

Science, this issue p. 787


In molecular collisions, resonances occur at specific energies at which the colliding particles temporarily form quasibound complexes, resulting in rapid variations in the energy dependence of scattering cross sections. Experimentally, it has proven challenging to observe such scattering resonances, especially in differential cross sections. We report the observation of resonance fingerprints in the state-to-state differential cross sections for inelastic NO-He collisions in the 13 to 19 centimeter–1 energy range with 0.3 centimeter–1 resolution. The observed structures were in excellent agreement with quantum scattering calculations. They were analyzed by separating the resonance contributions to the differential cross sections from the background through a partitioning of the multichannel scattering matrix. This revealed the partial-wave composition of the resonances and their evolution during the collision.

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