Molecular square dancing in CO-CO collisions

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Science  17 Jul 2020:
Vol. 369, Issue 6501, pp. 307-309
DOI: 10.1126/science.aan2729

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Molecule-molecule forward scattering

The classical picture of nonreactive inelastic scattering implies that a considerable transfer of translational to rotational energy should be accompanied by backward scattering. Using cross–molecular beam scattering combined with velocity map imaging, Sun et al. provide evidence of an unusual pair-correlated channel in the collision of carbon monoxide (CO) molecules, specifically 13CO with 12CO, both initially in the ground rotational state. These two molecules were scattered into the forward direction, both finally in the high rotational states. The experimental work is supported by quasiclassical trajectory simulations that exhibit unexpected molecular dance moves and suggest that similar effects could be expected for other molecule-molecule scattering systems.

Science this issue p. 307


Knowledge of rotational energy transfer (RET) involving carbon monoxide (CO) molecules is crucial for the interpretation of astrophysical data. As of now, our nearly perfect understanding of atom-molecule scattering shows that RET usually occurs by only a simple “bump” between partners. To advance molecular dynamics to the next step in complexity, we studied molecule-molecule scattering in great detail for collision between two CO molecules. Using advanced imaging methods and quasi-classical and fully quantum theory, we found that a synchronous movement can occur during CO-CO collisions, whereby a bump is followed by a move similar to a “do-si-do” in square dancing. This resulted in little angular deflection but high RET to both partners, a very unusual combination. The associated conditions suggest that this process can occur in other molecule-molecule systems.

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