Forward scattering due to slow-down of the intermediate in the H + HD → D + H₂ reaction
Steven A. Harich, Dongxu Dai, Chia C. Wang, Xueming Yang, Sheng Der Chao, and Rex T. Skodje
Nature 419, 281 (2002)

Quantum dynamical processes near the energy barrier that separates reactants from products influence the detailed mechanism by which elementary chemical reactions occur. In fact, these processes can change the product scattering behaviour from that expected from simple collision considerations, as seen in the two classical reactions F + H₂ à HF+ H and H + H₂à H₂ + H and their isotopic variants. In the case of the F + HD reaction, the role of a quantized trapped Feshbach resonance state had been directly determined, confirming previous conclusions that Feshbach resonances cause state-specific forward scattering of product molecules. Forward scattering has also been observed in the H + D₂àHD + D reaction and attributed to a time-delayed mechanism. But despite extensive experimental and theoretical investigations, the details of the mechanism remain unclear. Here we present crossed-beam scattering experiments and quantum calculations on the H + HDà H₂ +D reaction.  We find that the motion of the system along the reaction coordinate slows down as it approaches the top of the reaction barrier, thereby allowing vibrations perpendicular to the reaction coordinate and forward scattering. The reaction thus proceeds, as previously suggested, through a well-defined ‘quantized bottleneck state’ different from the trapped Feshbach resonance states observed before.