Department of Chemistry, Stanford University, Stanford, CA 94305-5080, USA.
Science. 2012 Jun 29;336(6089):1687-90. doi: 10.1126/science.1221329.
When a hydrogen (H) atom approaches a deuterium (D(2)) molecule, the minimum-energy path is for the three nuclei to line up. Consequently, nearly collinear collisions cause HD reaction products to be backscattered with low rotational excitation, whereas more glancing collisions yield sideways-scattered HD products with higher rotational excitation. Here we report that measured cross sections for the H + D(2) → HD(v' = 4, j') + D reaction at a collision energy of 1.97 electron volts contradict this behavior. The anomalous angular distributions match closely fully quantum mechanical calculations, and for the most part quasiclassical trajectory calculations. As the energy available in product recoil is reduced, a rotational barrier to reaction cuts off contributions from glancing collisions, causing high-j' HD products to become backward scattered.
当一个氢（H）原子接近氘（D(2)）分子时，三条核线对齐的路径是最低能量路径。因此，几乎共线的碰撞导致 HD 反应产物以低转动激发向后散射，而更掠射的碰撞则产生具有更高转动激发的侧向散射的 HD 产物。在这里，我们报告在 1.97 电子伏特的碰撞能下测量的 H + D(2) → HD(v' = 4, j') + D 反应的截面与这种行为相矛盾。异常的角分布与完全量子力学计算以及大部分准经典轨迹计算非常吻合。随着产物反冲中可用的能量减少，反应的转动势垒切断了掠射碰撞的贡献，导致高 j' HD 产物向后散射。
