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

Now physicists from the University of Innsbruck’s Roland Wester have observed a quantum mechanical tunneling reaction in experiments for the first time. It is the slowest charged particle reaction 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 for exact measurements and yet can be described quantum mechanically. The idea came to me 15 years ago while talking to a colleague at a conference in the US.”

The reaction is incredibly unlikely and slow due to the tunneling effect, making experimental observation extremely challenging. But for the first time, Wester’s team succeeded after several failed attempts.

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. Still, the collision of the two occasionally provokes a reaction.

The study’s first author, Robert Wild, said: “This is caused by the tunneling effect: quantum mechanics allows particles to break through the energetic barrier through 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 can benefit from the tunneling effect. For the first time, a measurement is now available that is also known in scientific theory. Based on this, scientists can make theoretical models of simpler chemical reactions and test them using the reaction that has already been proven to work.

For example, the scanning tunneling microscope and flash memories use 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 tunneling effect. The experiment of Wester’s team thus lays the foundation for a deeper understanding of various chemical processes.

Magazine 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