Harry Potter reaches platform 9 3/4 by going through a wall. What is impossible in everyday life is normal in the quantum realm: particles can overcome impenetrable barriers. This is known as “the tunnel”. Physicists from Innsbruck have now succeeded for the first time in experimentally observing a chemical tunnel reaction. As they report in the journal “Nature,” this is the slowest interaction ever observed with charged particles.
It has been known for nearly 100 years that a particle can overcome a barrier even if it does not have the necessary energy. The background to this “tunnel effect” is that in quantum mechanics, the position and velocity of a particle cannot be precisely determined at the same time. One can only make probabilistic data. Therefore, an atom, for example, can also be behind an energy barrier with a certain probability.
Technologically, this quantum effect is already being used in several ways, for example in scanning tunneling microscopy and in flash memory. Alpha decay of the atomic nucleus can also be explained by it.
The tunnel effect also plays a role in chemical reactions, for example when chemical reaction barriers are overcome as a result. However, it is very difficult to predict tunnel reactions. Since it is difficult to quantum mechanically accurately describe a chemical reaction with more than three particles, it is almost impossible with more than four particles.
In their work, Roland Wester and his team from the Institute of Ion Physics and Applied Physics at the University of Innsbruck investigated one of the fundamental molecular interactions – that is, between charged atomic hydrogen and molecular hydrogen. This simple interaction can be fully described in quantum mechanics.
To do this, they used deuterium, that is, an isotope of hydrogen with an extra neutron in the nucleus. This isotope is negatively charged, that is, to form an ion, it is placed in an ion trap and cooled to about minus 263 degrees Celsius. The trap is then filled with hydrogen gas, which consists of molecules with two hydrogen atoms each.
Tunnel effect in very rare cases
Despite the low temperatures, there are many collisions, but deuterium ions lack the energy to interact with hydrogen molecules in the traditional way. However, in very rare cases, this is caused by the tunneling effect: “Quantum mechanics allows particles to break through the energy barrier and a reaction occurs,” first author Robert Wilde of the Wester team explained in a broadcast. This reaction produces negatively charged hydrogen ions and deuterium hydrogen molecules.
“We let the experiment run for 15 minutes – much longer than is technically possible in most labs around the world for these ions,” Wester told APA. The researchers then determined the number of hydrogen ions formed, from which they could infer how often the reaction occurred. Over the long observation period, less than one percent of the particles reacted chemically—making it the slowest-reacting ion on record.
However, Wester stressed that slowness is relative, as it also depends on the amount of hydrogen being presented to deuterium ions as a partner in the reaction. So it would be more correct to talk about the probability of a collision reaction out of 100 billion – a tunneling reaction rarely happens. This value is consistent with one calculated by theoretical physicists in 2018. According to the researchers, this is the first time that an accurate theoretical model of the tunnel effect in a chemical reaction has been confirmed.
From this first measurement of the tunnel reaction, which is also well understood in theory, scientists hope to gain a better understanding of other chemical reactions behind which the tunnel effect is suspected.
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