title: 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.
authors.affiliation: 1] Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel [2].
authors.forename: Etay
authors.initials: E
authors.lastname: Lavert-Ofir
authors.affiliation: 1] Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel [2].
authors.forename: Yuval
authors.initials: Y
authors.lastname: Shagam
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Alon B
authors.initials: AB
authors.lastname: Henson
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Sasha
authors.initials: S
authors.lastname: Gersten
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742-2021, USA.
authors.forename: Jacek
authors.initials: J
authors.lastname: Kłos
authors.affiliation: Institute of Physics, Nicolaus Copernicus University, Grudziądzka 5, 87-100 Toruń, Poland.
authors.forename: Piotr S
authors.initials: PS
authors.lastname: Zuchowski
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Julia
authors.initials: J
authors.lastname: Narevicius
authors.affiliation: Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Edvardas
authors.initials: E
authors.lastname: Narevicius