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(2(3)P2), 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.
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
Yuval
Y
Shagam
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
Ayelet
A
Klein
Theoretische Physik, Universität Kassel, Heinrich-Plett-Straße 40, Kassel 34132, Germany.
Wojciech
W
Skomorowski
Department of Physics, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
Renjie
http://orcid.org/0000-0002-2141-9084
R
Yun
Department of Physics, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
Vitali
V
Averbukh
Theoretische Physik, Universität Kassel, Heinrich-Plett-Straße 40, Kassel 34132, Germany.
Christiane P
http://orcid.org/0000-0001-6285-5766
CP
Koch
Department of Chemical Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
Edvardas
E
Narevicius