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Molecular hydrogen interacts more strongly when rotationally excited at low temperatures leading to faster reactions


The role of internal molecular degrees of freedom, such as rotation, has scarcely been explored experimentally in low-energy collisions despite their significance to cold and ultracold chemistry. Particularly important to astrochemistry is the case of the most abundant molecule in interstellar space, hydrogen, for which two spin isomers have been detected, one of which exists in its rotational ground state whereas the other is rotationally excited. Here we demonstrate that quantization of molecular rotation plays a key role in cold reaction dynamics, where rotationally excited ortho-hydrogen reacts faster due to a stronger long-range attraction. We observe rotational state-dependent non-Arrhenius universal scaling laws in chemi-ionization reactions of para-H2 and ortho-H2 by He(23P2), spanning three orders of magnitude in temperature. Different scaling laws serve as a sensitive gauge that enables us to directly determine the exact nature of the long-range intermolecular interactions. Our results show that the quantum state of the molecular rotor determines whether or not anisotropic long-range interactions dominate cold collisions.

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Figure 1: Schematic of the experimental set-up.
Figure 2: Interaction potential energy surfaces.
Figure 3: Penning ionization widths.
Figure 4: Reaction rate constants for molecules in excited and ground rotational states with He(23P2) from 10 mK up to 300 K.


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The authors thank R. Kosloff and R. Moszynski for discussions as well as O. Tal and D. Rakhmilevitch for advice and help in the generation of para-hydrogen. This research was made possible, in part, by the historic generosity of the Harold Perlman family. The authors acknowledge financial support from the European Commission through ERC grant EU-FP7-ERC-CoG 1485 QuCC (Y.S., A.K., E.N.), from the Alexander von Humboldt Foundation (W.S.), from the Lee Family Foundation (R.Y.) and from the UK's Engineering and Physical Sciences Research Council (V.A., R.Y.) through the Career Acceleration Fellowship (award EP/H003657/1) and the Programme Grant on Attosecond Dynamics (award EP/I032517), as well as from the Deutsche Forschungsgemeinschaft through Research Unit 1789 (V.A.).

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The experimental work and data analysis were carried out by Y.S., A.K. and E.N. Ab initio potential surfaces and interaction strengths were calculated by W.S. The ionization widths were calculated by R.Y. and V.A. All authors contributed to the discussion of experimental results, derivation of the theoretical model and writing of the manuscript.

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Correspondence to Edvardas Narevicius.

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The authors declare no competing financial interests.

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Shagam, Y., Klein, A., Skomorowski, W. et al. Molecular hydrogen interacts more strongly when rotationally excited at low temperatures leading to faster reactions. Nature Chem 7, 921–926 (2015).

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