Vibrational excitation through tug-of-war inelastic collisions

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 scattered(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-2 (nu = 0, j = 0, 2) --> H + D-2 (nu' = 3, j' = 0, 2, 4, 6, 8) leads dominantly to forward scattering (nu and j respectively refer to the vibrational and rotational quantum numbers of the D-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-2 complex dissociates. We suggest that this 'tug of war' between H and D-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.