Quantum effects demonstrated in the collision of hydrogen molecules with noble gas atoms

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A research team of Freie Universität Berlin has shown how hydrogen molecules behave quantum mechanically when they collide with noble gas atoms such as helium or neon. In the study published in the journal "Science," the scientists used simulations to establish a direct link between measurements of atoms and molecules taken in experiments and theoretical models; the study includes both theoretical calculations and data from experiments conducted at the Technical University of Dortmund and the Weizmann Institute of Science (Israel). In the process, the team was able to show that the collision changes the way the molecules vibrate and spin in space according to the laws of quantum mechanics. Research on quantum physics is becoming increasingly significant. The findings are applied to cell phones, televisions, satellites and medical diagnostics, among other things.

The quantum effect now observed is a so-called Feshbach resonance. Here, a chemical bond is formed between the hydrogen molecule and the noble gas atom for a short time before the two colliding partners separate again," explains physicist Christiane Koch from Freie Universität Berlin.

Despite the extremely detailed measurements and calculations for a comparatively small and simple system, however, it is still far from being possible to capture all the information needed to reconstruct the complete quantum mechanical properties of the collision of a hydrogen molecule with a noble gas atom. This is due to a fundamental phenomenon of quantum mechanics: Namely, measurements represent the interface with classical physics. Our work thus illustrates the dilemma that we can grasp quantum mechanics mathematically in the abstract, but that we also need classical terms for a complete understanding," says Christiane Koch.

Quantum effects - behavior that cannot be understood with the help of classical physics - become apparent when atoms and molecules can no longer be described only by the place where they are located and the speed at which they move. They then also exhibit properties that we associate with the propagation of waves, for example in the form of interference, i.e. the constructive or destructive superposition of waves," explains Christiane Koch. In addition, there are phenomena such as entanglement, in which quantum mechanical objects instantaneously influence each other despite spatial distance.

Quantum effects typically occur when very small objects such as atoms and molecules are involved and the influence of the environment on these objects is very small. The latter is achieved at extremely short times or by extremely low temperatures close to absolute zero, i.e. minus 273.15 degrees Celsius. Only a small number of so-called quantum states are occupied, and the system behaves in an orderly fashion," says Christiane Koch.

At higher temperatures, more and more of the quantum mechanically permissible states are occupied and quantum mechanical effects disappear in the statistical averaging over all states. The system then behaves more randomly, i.e. it can be described on the basis of statistics. So far, even in the collisions at the coldest temperature, this statistical behavior has been observed in the collision of atoms and molecules. "Then it is difficult to impossible to draw conclusions from the measurement of the atoms and molecules about their interaction and thus to establish a direct link between experimental measurement and theoretical model," explains Christiane Koch. (acs)