1. Laser Chemistry, Spectroscopy and Dynamics Group, School of Chemistry, University of Bristol, Bristol BS8 1TS, UK
2. Department of Chemistry, University of Durham, Durham DH1 3LE, UK
3. Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA
4. These authors contributed equally to this work.

Correspondence to: Richard N. Zare³ Correspondence and requests for materials should be addressed to R.N.Z. (Email: zare@stanford.edu).

Abstract

Vibrationally inelastic scattering is a fundamental collision process that converts some of the kinetic energy of the colliding partners into vibrational excitation^1,2. 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.

1. Laser Chemistry, Spectroscopy and Dynamics Group, School of Chemistry, University of Bristol, Bristol BS8 1TS, UK
2. Department of Chemistry, University of Durham, Durham DH1 3LE, UK
3. Department of Chemistry, Stanford University, Stanford, California 94305-5080, USA
4. These authors contributed equally to this work.

Correspondence to: Richard N. Zare³ Correspondence and requests for materials should be addressed to R.N.Z. (Email: zare@stanford.edu).

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