Orii Takaaki, Okada Yoshiki, Takeuchi Kazuo, Ichihashi Masahiko, Kondow Tamotsu
RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.
J Chem Phys. 2004 Nov 15;121(19):9406-16. doi: 10.1063/1.1804179.
An incorporation of ND(3) into protonated ammonia cluster ions NH(4)(+)(NH(3))(n-1) (n=3-9), together with a dissociation of the cluster ions, was observed in the collision of the cluster with ND(3) at collision energies ranging from 0.04 to 1.4 eV in the center-of-mass frame. The branching fractions of the cluster ion species produced in the reactions were obtained as a function of the collision energy. The branching fractions of the incorporation products were successfully explained in terms of the Rice-Ramsperger-Kassel (RRK) theory at collision energies lower than the binding energy of the cluster ion. In addition, the internal energy distributions of the parent cluster ions were determined, and found to be in good agreement with those predicted using the evaporative ensemble model. In incorporations at collision energies lower than the binding energy of the cluster ion, all of the collision energy was transferred to the internal energy of the cluster ions; subsequently, an evaporation of ammonia molecules occurred in an equilibrium process after a complete energy redistribution in the clusters. In contrast, at collision energies higher than the binding energy of the cluster ion, a release of an ammonia molecule from the incorporation products occurred in a nonequilibrium process. The transition from the complex mode to the direct mode in the incorporation was observed at collision energies approximately equal to the binding energy. On the other hand, the collision energy dependence of the cross sections for the dissociation and for a nonreactive collision were estimated by a RRK simulation in which the collision energy transfer was interpreted by using the classical hard-sphere collision model. A relationship between reactivity and reaction modes in the collision of NH(4)(+)(NH(3))(4) with ND(3) is discussed via a comparison of the experimental results with the RRK simulation.
在质心参考系中，碰撞能量范围为0.04至1.4 eV时，观察到ND₃与质子化氨簇离子NH₄⁺(NH₃)ₙ₋₁（n = 3 - 9）发生碰撞时，ND₃并入簇离子，同时簇离子发生解离。反应中产生的簇离子种类的分支比作为碰撞能量的函数得到。在低于簇离子结合能的碰撞能量下，并入产物的分支比根据赖斯 - 拉姆斯佩格 - 卡斯尔（RRK）理论得到了成功解释。此外，确定了母体簇离子的内能分布，发现其与使用蒸发系综模型预测的结果高度吻合。在低于簇离子结合能的碰撞能量下进行并入时，所有碰撞能量都转移到了簇离子的内能上；随后，在簇内完全能量重新分布后，氨分子在平衡过程中发生蒸发。相比之下，在高于簇离子结合能的碰撞能量下，并入产物中氨分子的释放发生在非平衡过程中。在碰撞能量近似等于结合能时，观察到并入过程从复合模式向直接模式的转变。另一方面，通过RRK模拟估计了解离和非反应性碰撞截面的碰撞能量依赖性，其中碰撞能量转移使用经典硬球碰撞模型进行解释。通过将实验结果与RRK模拟进行比较，讨论了NH₄⁺(NH₃)₄与ND₃碰撞中反应性与反应模式之间的关系。
