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.

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.