Photo © Max Planck Institute for Quantum Optics
Physicists empirically for the first time confirmed the presence of a quantum tunnel effect in chemical reactions. This is stated in an article by scientists from the Institute of Ion Physics and Applied Physics at the University of Innsbruck in Austria. They experimentally verified that chemical transformations forbidden from the point of view of classical physics can occur due to quantum effects.
Imagine that we have a fence in front of us. It can be jumped over if we have enough energy. This is classical physics. However, everyone will be surprised if you (or an elementary particle), having no energy to jump over the fence, instantly find yourself on the other side of it. In this case, one speaks of quantum tunneling. Intuitively, it can be understood as follows: in the quantum world, all objects are “smeared” – we are not talking about the position of a particle or its speed, but about its wave function, which allows us to calculate the probability of finding a particle. The probability of finding a particle behind the fence is not equal to zero, so sooner or later we will definitely find it there.
The phenomenon of tunneling has been theoretically known for about a hundred years. An important achievement of the tunnel effect was the correct explanation of alpha decay processes in nuclear physics. But in chemical reactions with ions, the phenomenon of tunneling has never been observed. In the macroscopic world, as a rule, it is impossible to directly observe the quantum properties of objects. The more significant each new observation is when classical physics “collides” with quantum physics.
A team of Austrian scientists led by Roland Vester did this using the example of a chemical reaction in which a negative deuterium ion reacts with a hydrogen molecule, forming a negative hydrogen ion and a “deuterium-hydrogen” molecule. (Recall that deuterium is a “heavy hydrogen” atom, consisting of one proton and one neutron). Classically, such a reaction is forbidden at the temperatures at which the researchers tried to conduct it. However, due to the tunneling effect, deuterium and a hydrogen atom can change places.
This reaction was chosen for study for two reasons. First, this is one of the simplest chemical reactions for theoretical calculation, in which tunneling can be expected. Secondly, hydrogen is the most common element in the Universe, and even if the reaction due to tunneling proceeds at a very low rate, on a cosmic scale we will notice its consequences, for example, in clouds of cold interstellar gas.
To test whether the classically impracticable reaction is indeed possible due to quantum phenomena, physicists caught and cooled deuterium ions in a cryogenic radio frequency trap, mixed it with molecular hydrogen at a temperature of 15 Kelvin (this is minus 258 degrees Celsius) and waited. The resulting reaction products were studied on a time-of-flight mass analyzer (this device, by accelerating ions by an electric field, makes it possible to determine the ratio of the ion mass to its charge from their velocity). The time-of-flight mass spectrum plot shows a small peak corresponding to negative ordinary hydrogen ions. This means that a “forbidden” reaction has taken place. After waiting 1000 seconds, the scientists were able to get about 1% of the ions to react; hence it follows that every one hundred billionth deuterium ion reacts upon collision with a hydrogen molecule. This exactly corresponded to the calculations made on the basis of the tunnel effect.