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Observation of correlated excitations in bimolecular collisions


Although collisions between atoms and molecules are largely understood, collisions between two molecules have proven much harder to study. In both experiment and theory, our ability to determine quantum-state-resolved bimolecular cross-sections lags behind their atom–molecule counterparts by decades. For many bimolecular systems, even rules of thumb—much less intuitive understanding—of scattering cross sections are lacking. Here, we report the measurement of state-to-state differential cross sections on the collision of state-selected and velocity-controlled nitric oxide (NO) radicals and oxygen (O2) molecules. Using velocity map imaging of the scattered NO radicals, the full product-pair correlations of rotational excitation that occurs in both collision partners from individual encounters are revealed. The correlated cross sections show surprisingly good agreement with quantum scattering calculations using ab initio NO−O2 potential energy surfaces. The observations show that the well-known energy-gap law that governs atom–molecule collisions does not generally apply to bimolecular excitation processes, and reveal a propensity rule for the vector correlation of product angular momenta.

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Fig. 1: Rotational energy level diagrams of NO and O2, and the Newton diagram for NO–O2 collisions.
Fig. 2: Experimental (Exp.) and simulated (Sim.) scattering images for the scattering processes NO (1/2f) + O2 (\({N}_{{{\rm{O}}}_{2}}=1\)) → NO (\({j}_{{\rm{N}}{\rm{O}}}^{{}^{<mml:mpadded xmlns:xlink="" voffset="-4pt">{\boldsymbol{\text{'}}}</mml:mpadded>}}\)) + O2 (\({N}_{{{\rm{O}}}_{2}}^{<mml:mpadded xmlns:xlink="" voffset="-1pt">\text{'}</mml:mpadded>}\)).
Fig. 3: Radial intensity distribution of the experimental (red curves) and simulated (blue curves) scattering images from Fig. 2.
Fig. 4: Analysis of the sum of angular momenta before and after the collision.


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The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013), ERC grant agreement 335646 MOLBIL. This work is part of the research program of the Netherlands Organization for Scientific Research (NWO). The expert technical support by N. Janssen, A. van Roij and E. Sweers is gratefully acknowledged.

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Authors and Affiliations



The experiments were conceived by S.Y.T.v.d.M. The experiments were carried out by Z.G. with the help of S.N.V. Data analysis and simulations were performed by Z.G. Potential energy surfaces were calculated by T.K., A.v.d.A. and G.C.G. Scattering calculations were performed by T.K. and M.B. Analysis of quantum entanglement was performed by T.K. All authors were involved in the interpretation of the data, discussed the results, and commented on the manuscript. The paper was written by Z.G., T.K. and S.Y.T.v.d.M. with contributions from all authors.

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Correspondence to Gerrit C. Groenenboom or Sebastiaan Y. T. van de Meerakker.

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Supplementary Methods, Analysis and Calculations; Supplementary Figs. 1–14

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Gao, Z., Karman, T., Vogels, S.N. et al. Observation of correlated excitations in bimolecular collisions. Nature Chem 10, 469–473 (2018).

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