Atomic tug of war

When an atom collides with a molecule, the two can recoil off each other like billiard balls. But, as Stuart Greaves (University of Bristol, UK) and colleagues Richard Zare (Stanford University) and Eckart Wrede (Durham University, UK) have discovered, the atom and molecule can also pull on each other. The researchers examined collisions of atomic hydrogen with molecular deuterium (a favorite system for testing the most intricate predictions of quantum chemistry), focusing on collisions in which the D₂ molecule emerged intact but with three additional quanta of vibrational energy. The expectation was that such vibrational excitation requires a head-on collision, from which the products are backscattered. However, Greaves and colleagues found that in many of the collisions they looked at, the atom and molecule were forward scattered. Simulations of the curious collisions revealed that the D₂ bond is lengthened, not compressed, under the influence of the H atom. The atom and molecule undergo what is essentially a frustrated chemical reaction: Instead of forming HD + D, they recross the reaction barrier back to H + D₂. It seems that a transitory chemical bond forms between the H atom and one of the D atoms, which tugs on the D-D bond and sets it vibrating before the H atom continues on its way. (S. J. Greaves et al., Nature 454 , 88, 2008 http://dx.doi.org/10.1038/nature07079 .)