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Observation of the isotope effect in sub-kelvin reactions

Abstract

Quantum phenomena in the translational motion of reactants, which are usually negligible at room temperature, can dominate reaction dynamics at low temperatures. In such cold conditions, even the weak centrifugal force is enough to create a potential barrier that keeps reactants separated. However, reactions may still proceed through tunnelling because, at low temperatures, wave-like properties become important. At certain de Broglie wavelengths, the colliding particles can become trapped in long-lived metastable scattering states, leading to sharp increases in the total reaction rate. Here, we show that these metastable states are responsible for a dramatic, order-of-magnitude-strong, quantum kinetic isotope effect by measuring the absolute Penning ionization reaction rates between hydrogen isotopologues and metastable helium down to 0.01 K. We demonstrate that measurements of a single isotope are insufficient to constrain ab initio calculations, making the kinetic isotope effect in the cold regime necessary to remove ambiguity among possible potential energy surfaces.

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Figure 1: Plot of effective potential surface and wavefunctions at different energies.
Figure 2: Schematic of the merged supersonic beam configuration.
Figure 3: Penning ionization reaction rates of metastable helium, He*(23S) and the hydrogen isotopologues H2, HD and D2.
Figure 4: Sensitivity analysis of the He*–H2 reaction.

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Acknowledgements

The authors thank U. Even and Y. Prior for discussions. The authors thank J.W. Rosenberg for reading the manuscript. This research was made possible, in part, by the historic generosity of the Harold Perlman family. E.N. acknowledges support from the Israel Science Foundation and the Minerva Foundation. J.K. acknowledges financial support through the United States National Science Foundation (grant no. CHE-1213332) to M. Alexander. P.S.Z. was supported by the Iuventus Plus grant by the Polish Ministry of Science and Higher Education.

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Contributions

The experimental work and data analysis were carried out by E.L-O., Y.S., A.B.H., S.G., J.N. and E.N. The ab initio potential surfaces were calculated by J.K. and P.S.Z.

Corresponding author

Correspondence to Edvardas Narevicius.

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The authors declare no competing financial interests.

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Lavert-Ofir, E., Shagam, Y., Henson, A. et al. Observation of the isotope effect in sub-kelvin reactions. Nature Chem 6, 332–335 (2014). https://doi.org/10.1038/nchem.1857

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