For the first time, quantum mechanical tunneling reaction observed in experiments

Quan­tum Chem­istry: Molecules caught tun­nel­ing.


Quantum tunneling reactions are essential in chemistry when classical pathways are energetically forbidden. Such reactions are quite challenging to calculate theoretically because of the high dimensionality of quantum dynamics. They also are challenging to identify in experiments.

Now, for the first time, physicists at the Roland Wester of the University of Innsbruck observed a quantum mechanical tunneling reaction in experiments. It is the slowest reaction with charged particles ever observed.

Roland Wester from the Department of Ion Physics and Applied Physics at the University of Innsbruck said, “It requires an experiment that allows exact measurements and can still be described quantum-mechanically. The idea came to me 15 years ago in a conversation with a colleague at a conference in the U.S.”

The reaction is incredibly unlikely and slow due to the tunnel effect, making experimental observation extremely challenging. Yet for the first time, Wester’s team has succeeded in doing that after multiple failed tries.

They used hydrogen for the experiment. Later, they used deuterium in an ion trap, cooled it down, and filled it with hydrogen gas.

Low temperatures prevent the negatively charged deuterium ions from reacting with hydrogen molecules in a typical manner. Yet, the collision of the two does occasionally cause a reaction.

The study’s first author, Robert Wild, said“The tunnel effect causes this: Quantum mechanics allows particles to break through the energetic barrier due to their quantum mechanical wave properties, and a reaction occurs. In our experiment, we give possible reactions in the trap for about 15 minutes and then determine the number of hydrogen ions formed. From their number, we can deduce how often a reaction has occurred.”

Several chemical processes might benefit from the tunnel effect. For the first time, a measurement that is also well-known in scientific theory is now available. Based on this, scientists can create theoretical models of simpler chemical reactions and test them using the reaction that has already been proven to work.

For instance, the scanning tunneling microscope and flash memories exploit the tunnel effect. The tunnel effect also explains the alpha decay of atomic nuclei. Certain astrochemical syntheses of molecules in dark interstellar clouds can also be described by the tunnel effect. So, the experiment conducted by Wester’s team sets the groundwork for a deeper comprehension of several chemical processes.

Journal Reference:

  1. Robert Wild, Markus Nötzold, Malcolm Simpson, Thuy Dung Tran, and Roland Wester. Tunneling measured in a very slow ion-molecule reaction. Nature 2023 DOI: 10.1038/s41586-023-05727-z
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