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Vibrational excitation through tug-of-war inelastic collisions

Nature volume 454, pages 88–91 (2008)Cite this article

Abstract

Vibrationally inelastic scattering is a fundamental collision process that converts some of the kinetic energy of the colliding partners into vibrational excitation1,2. The conventional wisdom is that collisions with high impact parameters (where the partners only ‘graze’ each other) are forward scattered and essentially elastic, whereas collisions with low impact parameters transfer a large amount of energy into vibrations and are mainly back scattered3. Here we report experimental observations of exactly the opposite behaviour for the simplest and most studied of all neutral–neutral collisions: we find that the inelastic scattering process H + D2(v = 0, j = 0, 2) → H + D2(v′ = 3, j′ = 0, 2, 4, 6, 8) leads dominantly to forward scattering (v and j respectively refer to the vibrational and rotational quantum numbers of the D2 molecule). Quasi-classical trajectory calculations show that the vibrational excitation is caused by extension, not compression, of the D–D bond through interaction with the passing H atom. However, the H–D interaction never becomes strong enough for capture of the H atom before it departs with diminished kinetic energy; that is, the inelastic scattering process is essentially a frustrated reaction in which the collision typically excites the outward-going half of the H–D–D symmetric stretch before the H–D2 complex dissociates. We suggest that this ‘tug of war’ between H and D2 is a new mechanism for vibrational excitation that should play a role in all neutral–neutral collisions where strong attraction can develop between the collision partners.

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Figure 1: Products of inelastic H + D 2 collisions are mostly forward scattered.
Figure 2: Impact parameter is linearly correlated with deflection angle.
Figure 3: Snapshots from a representative forward-scattered trajectory.

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Acknowledgements

The Stanford team gratefully acknowledges support by the US National Science Foundation under grant NSF CHE 0650414. S.J.G. is supported by the EPSRC LASER Portfolio Partnership grant GR/S71750/01.

Author Contributions N.T.G. designed and performed the experiments with the help of J.Z. and D.J.M., analysed the experimental data, and assisted with the manuscript. S.J.G and E.W. performed quasi-classical trajectory calculations, formulated the mechanism, and prepared figures and movies. R.N.Z. wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Author notes

  1. Stuart J. Greaves and Noah T. Goldberg: These authors contributed equally to this work.

Authors and Affiliations

  1. Laser Chemistry, Spectroscopy and Dynamics Group, School of Chemistry, University of Bristol, Bristol BS8 1TS, UK

    Stuart J. Greaves

  2. Department of Chemistry, University of Durham, Durham DH1 3LE, UK

    Eckart Wrede

  3. Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA,

    Noah T. Goldberg, Jianyang Zhang, Daniel J. Miller & Richard N. Zare

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  1. Stuart J. Greaves
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  6. Richard N. Zare
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Corresponding author

Correspondence to Richard N. Zare.

Supplementary information

Supplementary information

The file contains Supplementary Figures 1-7 with Legends. The file includes an overview of the QCT method and the BKMP2 PES, followed by impact-parameter/deflection-angle correlation diagrams indicating those trajectories for which movies are made available in the Supplementary Information. Measured DCSs for the complementary reactive HD(v?=3, j?=0) channel are compared with fully QM calculations. (PDF 631 kb)

Supplementary information

The file contains Supplementary Videos 1-6. The file includes QCT movies for six representative trajectories leading to D2(v?=3, j?=0) products. In each case the D-D bond is not compressed by the incoming H atom; instead, it is extended. In some cases it is pulled out whereas in others it is stabilized at the turning point. (PPT 4558 kb)

Supplementary information

The file contains Supplementary Videos 7-10. The file includes QCT movies for four representative trajectories leading to D2(v?=3, j?=4) products. The same mechanism is operative for this product state: the D-D bond is not compressed by the incoming H atom; instead it is extended. In some cases it is pulled out whereas in others it is stabilized at the turning point. (PPT 4570 kb)

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Greaves, S., Wrede, E., Goldberg, N. et al. Vibrational excitation through tug-of-war inelastic collisions. Nature 454, 88–91 (2008). https://doi.org/10.1038/nature07079

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