International Collaboration in Chemistry: Quantum Dynamics of 4-Atom Bimolecular Reactions

拟议的研究将建立在R.N. Zare（斯坦福大学）的实验小组和S.C. Althorpe（剑桥大学）的理论小组之间最近的一系列合作的基础上。在这项工作中，我们在实验和理论上以前所未有的细节和严格程度研究了最简单的化学反应（H + H2 -> H2 + H和同位素变化），使用时变波包来解释实验散射数据。令人惊讶的是，这揭示了由氢核的量子特性引起的各种不同寻常的机械效应，并对其他广泛的化学反应产生了影响。本提案的目的是将这些研究扩展到最简单的4原子反应之一：H + H2O -> H2 + OH。额外的原子的意义在于，它允许多种3原子反应（如H + H2）显然太简单而无法捕获的效应，例如立体动力学过程，以及模式选择性对过渡态波函数的影响。这次合作的理论部分将扩展到4个原子，即Althorpe和同事的“平面波包方法”，其中反应产物在空间中的散射是通过时间演变的波包来描述的，可以直接映射到实验测量的状态到状态的微分截面上。波包将使用一种新的4原子反应散射代码进行计算，该代码最近在CCP6（英国分子量子动力学协同计算项目）的支持下开发。横截面将由Zare小组测量，使用一种新建造的仪器的扩展，用于成像光引发化学反应的光电离产物的三维速度分布。通过以这种方式将理论和实验结合起来，我们将能够比单独应用任何一种方法更深入地探索动力学。特别是，实验数据将用于扫描截面，以便昂贵的计算可以集中在那些最有可能对应有趣的动态效应的最终状态上。实验数据之间的信息交换也将用于减少需要执行的计算次数（通过插值），理论结果反过来将允许从原始实验数据中提取更精确的角度依赖性测量。我们有信心，这一策略将导致第一个描述4原子反应产物散射到空间的时变波函数，其计算成本远低于单独理论作用所能实现的计算成本。

The proposed research will build on a recent series of collaborations between the experimental group of R.N. Zare (Stanford University) and the theoretical group of S.C. Althorpe (University of Cambridge). In this work, we have studied the simplest chemical reaction (H + H2 -> H2 + H and isotopic variants) at an unprecedented level of detail and rigor, both experimentally and theoretically, using time-dependent wave packets to interpret the experimental scattering data. Surprisingly, this has revealed a variety of unusual mechanistic effects that result from the quantum properties of the hydrogen nuclei, and which have implications for a wide range of other chemical reactions. The aim of this proposal is to extend these studies to one of the simplest 4-atom reactions: H + H2O -> H2 + OH. The significance of the extra atom is that it allows for a variety of effects that a 3-atom reaction such as H + H2 is obviously too simple to capture, such as stereodynamical processes, and the effect of mode-selectivity on the wave function at the transition state.The theoretical part of this collaboration will extend to 4-atoms the 'plane wave packet method' of Althorpe and co-workers, in which the scattering into space of the products of a reaction is described by time-evolving wave packets which can be mapped directly onto experimentally measured state-to-state differential cross sections. The wave packets will be computed using a new 4-atom reactive scattering code, developed recently under the aegis of CCP6 (UK Collaborative Computational Project on Molecular Quantum Dynamics). The cross sections will be measured by the Zare group, using an extension of a newly constructed instrument for imaging the three-dimensional velocity distribution of photoionized products from photoinitiated chemical reactions. By bringing together theory and experiment in this way, we will be able to probe much more deeply into the dynamics than would be possible if either were applied independently. In particular, the experimental data will be used to scan the cross sections, so that the expensive computations can focus on those final states that are most likely to correspond to interesting dynamical effects. The exchange of information between experimental data will also be used to reduce the number of calculations that need to be carried out (by interpolation), and the theoretical results will in turn allow more precise measurements of the angular dependence to be extracted from the raw experimental data. We are confident that this strategy will result in the first time-dependent wave functions describing the scattering into space of products from a 4-atom reaction, at a much lower computational cost than could be achieved by theory acting alone.