Université de Lyon, F-69003, Lyon, France.
Phys Chem Chem Phys. 2018 Jul 14;20(26):18066-18073. doi: 10.1039/c8cp02657b. Epub 2018 Jun 22.
Velocity distributions of neutral water molecules evaporated after collision induced dissociation of protonated water clusters H(HO) were measured using the combined correlated ion and neutral fragment time-of-flight (COINTOF) and velocity map imaging (VMI) techniques. As observed previously, all measured velocity distributions exhibit two contributions, with a low velocity part identified by statistical molecular dynamics (SMD) simulations as events obeying the Maxwell-Boltzmann statistics and a high velocity contribution corresponding to non-ergodic events in which energy redistribution is incomplete. In contrast to earlier studies, where the evaporation of a single molecule was probed, the present study is concerned with events involving the evaporation of up to five water molecules. In particular, we discuss here in detail the cases of two and three evaporated molecules. Evaporation of several water molecules after CID can be interpreted in general as a sequential evaporation process. In addition to the SMD calculations, a Monte Carlo (MC) based simulation was developed allowing the reconstruction of the velocity distribution produced by the evaporation of m molecules from H(HO) cluster ions using the measured velocity distributions for singly evaporated molecules as the input. The observed broadening of the low-velocity part of the distributions for the evaporation of two and three molecules as compared to the width for the evaporation of a single molecule results from the cumulative recoil velocity of the successive ion residues as well as the intrinsically broader distributions for decreasingly smaller parent clusters. Further MC simulations were carried out assuming that a certain proportion of non-ergodic events is responsible for the first evaporation in such a sequential evaporation series, thereby allowing to model the entire velocity distribution.
利用组合相关离子和中性碎片飞行时间（COINTOF）和速度映射成像（VMI）技术，测量了质子化水分子团 H(HO)碰撞诱导解离后蒸发的中性水分子的速度分布。如前所述，所有测量的速度分布都表现出两个贡献，其中一个低速度部分通过统计分子动力学（SMD）模拟被识别为符合麦克斯韦-玻尔兹曼统计的事件，而另一个高速度贡献则对应于能量重新分配不完全的非遍历事件。与早期研究中仅探测单个分子的蒸发相比，本研究涉及的是涉及蒸发多达五个水分子的事件。特别是，我们在这里详细讨论了蒸发两个和三个水分子的情况。CID 后蒸发几个水分子通常可以解释为顺序蒸发过程。除了 SMD 计算外，还开发了一种基于蒙特卡罗（MC）的模拟方法，允许使用测量的单蒸发分子的速度分布作为输入，重建由 H(HO)簇离子中蒸发 m 个分子产生的速度分布。与蒸发单个分子相比，蒸发两个和三个分子的分布的低速度部分的展宽来自于连续离子残基的累积反冲速度以及母体簇越来越小的固有更宽分布。进一步的 MC 模拟假设一定比例的非遍历事件负责此类顺序蒸发系列中的第一次蒸发，从而可以对整个速度分布进行建模。
