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Quantum-state-controlled channel branching in cold Ne(3P2)+Ar chemi-ionization

Abstract

A prerequisite to gain a complete understanding of the most basic aspects of chemical reactions is the ability to perform experiments with complete control over the reactant degrees of freedom. By controlling these, details of a reaction mechanism can be investigated and ultimately manipulated. Here, we present a study of chemi-ionization—a fundamental energy-transfer reaction—under completely controlled conditions. The collision energy of the reagents was tuned from 0.02 K to 1,000 K, with the orientation of the excited Ne atom relative to Ar fully specified by an external magnetic field. Chemi-ionization of Ne(3P2) and Ar in these conditions enables a detailed investigation of how the reaction proceeds, and provides us with a means to control the branching ratio between the two possible reaction outcomes. The merged-beam experimental technique used here allows access to a low-energy regime in which the atoms dynamically reorient into a favourable configuration for reaction, irrespective of their initial orientations.

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Fig. 1: Overview of the experimental apparatus that shows how the beam speeds affect the collision energy and how this is related spatially to the magnetic field.
Fig. 2: An example of mass spectra used to probe the steric effect and scan over the collision energy range.
Fig. 3: Experimental (red) state-dependent reactivities that quantify the steric effect are compared with theory (black).
Fig. 4: Ion yields of the AI and PI channels at different collision energies, Ecoll = 575 K (top), 175 K (middle) and 22 mK (bottom).
Fig. 5: The steric effect rapidly diminishes below 20 K and is lost long before the orientation angle becomes scrambled.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work is funded by EPFL and the Swiss Science Foundation (project number 200021_165975). J.J.O. and P.B. acknowledge the support by Natural Sciences and Engineering Research Council Canada.

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Contributions

The experiment was conceived by A.O. and S.D.S.G. The experiment and data analysis were performed by S.D.S.G. The potential surfaces and observables were calculated by J.J.O. and P.B. All the authors contributed to the interpretation of the data and discussion of the results. The manuscript was written by S.D.S.G., A.O. and J.J.O. with additional comments and contributions from all the authors.

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Correspondence to Andreas Osterwalder.

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Supplementary Information

Supplementary Methods, Supplementary Figures 1–6

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Gordon, S.D.S., Omiste, J.J., Zou, J. et al. Quantum-state-controlled channel branching in cold Ne(3P2)+Ar chemi-ionization. Nature Chem 10, 1190–1195 (2018). https://doi.org/10.1038/s41557-018-0152-2

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