Vibrationally inelastic H + D2 collisions are forward-scattered

We have measured differential cross sections (DCSs) for the vibrationally inelastic scattering process H + o-D-2(v = 0, j = 0,2) -> H + o-D-2(v' = 1-4, j' even). Several different collision energies and nearly the entire range of populated product quantum states are studied. The products are dominantly forward-scattered in all cases. This behavior is the opposite of what is predicted by the conventional textbook mechanism, in which collisions at small impact parameters compress the bond and cause the products to recoil in the backward direction. Recent quasiclassical trajectory (QCT) calculations examining only the o-D-2(v' = 3, j') products suggest that vibrationally inelastic scattering is the result of a frustrated reaction in which the D-D bond is stretched, but not broken, during the collision. These QCT calculations provide a qualitative explanation for the observed forward-scattering, but they do not agree with experiments at the lowest values of j'. The present work shows that quantum mechanical calculations agree closely with experiments and expands upon previous results to show that forward-scattering is universally observed in vibrationally inelastic H + D-2 collisions over a broad range of conditions.