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

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Richard Zare of Stanford University is supported by an International Collaborations in Chemistry (ICC) award from the Chemical Structure and Mechanisms program in the Chemistry division to carry out detailed studies of the quantum dynamics of four-atom bimolecular reaction, in collaboration with Stuart Althorpe of Cambridge University in the UK. The proposed research builds on a recent series of collaborations between Zare's experimental research group and the theoretical group of S.C. Althorpe in which they 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 research 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 current project extends these studies to one of the simplest four-atom reactions: H + H2O - H2 + OH. The significance of the extra atom is that it allows for a variety of effects that a three-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 extends to four 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 cross sections are measured by the Zare group, using an extension of a newly constructed instrument for imaging the three-dimensional velocity distribution of photo-ionized products from photo-initiated chemical reactions. An important feature of this research is that both highly accurate theory and novel and challenging experimental techniques are required; it is truly cutting edge research. This fundamental research, like the prior work on the simplest three atom reaction, is expected to lead to surprising results that will influence our perceptions of reaction mechanisms and reaction dynamics. The work gives a deeper understanding of the role that quantum mechanics plays in many reactive processes. The work is also likely to find its way into textbooks on physical chemistry, and influence how students and their teachers think about chemical reaction dynamics. Several graduate students, in the US and the UK are being trained in the leading-edge theoretical and experimental techniques as part on an international collaboration in which there is a high level of synergy between theory and experiment.