After fifteen years of experimentation, a group of researchers has succeeded in proving theoretical predictions about the quantum behavior of molecules. They observed for the first time a so-called tunnel effect, in which an atom dives into a molecule with a quantum leap.
The tunnel effect, or quantum tunneling, is an elusive, rare, and difficult to predict phenomenon. The laws of quantum mechanics allow atoms to sometimes overcome an energy barrier that is actually too high for them. The effect can sometimes enable a chemical reaction for which not enough energy is actually available.
A research group from Austria has now observed this tunnel effect for the first time in a chemical reaction for which an accurate calculation was also possible. Theorists can breathe a sigh of relief. The results support their predictions.
READ ALSO
Base math problems on social issues
When math is linked to questions that are relevant to students, it can …
Quantumtunneleffect
Such predictions are based on quantum mechanical simulations of reactions, which quickly become extremely complex. If three or four particles – atoms and/or molecules – are involved in a reaction, it is just a matter of time for theorists to calculate when the tunneling effect will occur. But if more than four particles are involved, the calculations are so complex that a theoretical prediction is almost impossible.
That’s why scientists normally simulate chemical reactions with computer models that largely ignore quantum effects. But is that always right? Do scientists understand quantum effects well enough to construct such models?
The experiment
To determine this, an experiment is needed in which the tunnel effect occurs, is easily measurable and the reaction is simple enough to accurately calculate the quantum mechanical effects.
‘During a conversation with a colleague, we came up with the idea for the experiment’, physicist Roland Wester of the University of Innsbruck in Austria emails. ‘We chose the reaction of negatively charged deuterium (a heavy variant of hydrogen, DS) with hydrogen gas.’ Hydrogen molecules consist of two hydrogen atoms.
In the experiment, Wester and his colleagues trapped the deuterium and cooled it down to -253 degrees Celsius. Then they added hydrogen gas. Normally, the two react with each other to form free, negatively charged hydrogen atoms and molecules consisting of deuterium and hydrogen. But at the low temperature in the experiment, the deuterium particles don’t have enough energy for that reaction.
The only way in which the deuterium-hydrogen molecules can form is via the rare quantum tunnel effect, in which the deuterium particles can make a quantum leap very occasionally. In this way they overcome the energy barrier, and they can still react with the hydrogen gas.
15 years of experimentation
Those quantum leaps are rare. To determine how often they occur, the researchers brought together several deuterium particles and hydrogen molecules. Then they waited 15 minutes and looked at how many free, negatively charged hydrogen atoms had formed. That indicated how many quantum leaps had taken place.
Put this way, this experiment sounds fairly simple: bring deuterium and hydrogen gas together, wait 15 minutes, and count the hydrogen atoms. But it took 15 years to build and fine-tune the experiment to be sensitive enough to measure this. Now that has finally happened. The results confirm the quantum mechanical calculations that had been awaiting the outcome of the experiment since 2018.
Wester: ‘Our measurements have shown that the calculations are indeed accurate. This gives confidence that such calculations can also be performed for other systems in order to provide a broader insight into the role of quantum tunneling in chemistry.’