Forward scattering due to slow-down of the intermediate in the H + HD → D + H₂ reaction
在 H + HD ® D + H₂ 的反應中，反應過程減緩而導致的前向散射產物

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.　

化學反應中，過渡態的量子動態的行為，可能導致產物的能態分佈或散射角度分佈，有別於碰撞理論的預測結果。我們探討 F + HD ® HF + D 的反應時，發現有Feshbach共振態，導致特別的前向散射產物。精密的量子化學計算指出，此現象是由於原子質點的運動在過渡態上因位能的升高而變慢，當反應經過這些量子化的瓶頸能態時，時間的延遲，使得產物的散射方向能夠轉至前向。