Vibrational excitation through tug-of-war inelastic collisions

Tugging the hydrogen atoms A simple and much-studied example of vibrationally inelastic collisions is the crashing of a hydrogen atom into a deuterium molecule. The conventional view of the process is that transient compression of the D-D bond gives rise to vibrational excitation, then the colliding partners scatter backwards. Surprisingly, experiments now reveal a different inelastic scattering mechanism. Greaves et al. observed vibrational excitation even in collisions where the two species merely graze each other. They attribute this to extension of the D-D bond through interaction with the passing H atom. This tug-of-war mechanism should be at play whenever attraction can develop between colliding partners. A simple and much-studied example of vibrationally inelastic collisions is the crashing of a hydrogen atom into a deuterium molecule. This experiment reveals a different inelastic scattering mechanism: it observed vibrational excitation even in collisions where the two species merely graze each other, and which is attributed to extension of the D-D bond through interaction with the passing H atom. This tug of war mechanism should be at play whenever attraction can develop between the colliding partners. Vibrationally inelastic scattering is a fundamental collision process that converts some of the kinetic energy of the colliding partners into vibrational excitation.sup.1.sup.,.sup.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 scattered.sup.3. 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 + D.sub.2(v = 0, j = 0, 2) [right arrow] H + D.sub.2(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 D.sub.2 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-D.sub.2 complex dissociates. We suggest that this 'tug of war' between H and D.sub.2 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.
Author(s): Stuart J. Greaves [sup.1] , Eckart Wrede [sup.2] , Noah T. Goldberg [sup.3] , Jianyang Zhang [sup.3] , Daniel J. Miller [sup.3] , Richard N. Zare [sup.3] Author Affiliations: [...]