There are two different ways the helium atom can be ionized.
Scientists have observed how quantum superpositions build up in a helium atom within femtoseconds. Just like the two paths through the plate in the double slit experiment, the ionization of helium can happen via 2 different processes at the same time, and this leads to characteristic interference effects. In the case of the helium atom, they are called “Fano resonances.” A team from TU Wien (Vienna), Max-Planck Institute for Nuclear Physics in Heidelberg and Kansas State University has now managed to observe the buildup up of these Fano resonances.
When a laser pulse transfers enough energy to one of the electrons in the helium atom, the electron is ripped out of the atom right away. There is, however, another way to ionize the helium atom, which is a little bit more complex, as Professor Joachim Burgdörfer explains: “If at first the laser lifts both electrons to a state of higher energy, one of the electrons may return into the state of lower energy. Part of this electron’s energy is transferred to the second electron, which can then leave the helium atom.” The outcome of both processes is the same – both turn the neutral He atom into an ion with 1 remaining electron.
“According to the laws of quantum physics, each atom can undergo both processes at the same time,” says Renate Pazourek . “And this combination of paths leaves us characteristic traces that can be detected.” Analyzing the light absorbed by the He atoms, Fano resonances are found – an unmistakable sign that the final state was reached via 2 different paths. This can also be prevented. During the ionization process, the indirect path can be effectively switched off with a second laser beam so that only the other path remains open and the Fano-resonance disappears.
This opens up a new possibility of studying the time evolution of this process. At first, the atom is allowed to follow both paths simultaneously. After some time, the indirect path is blocked. Depending on how long the system was allowed to access both paths, the Fano-resonance becomes more or less distinct.
“Fano resonances have been observed in a wide variety of physical systems, they play an important role in atomic physics,” says Stefan Donsa. “For the first time, it is now possible to control these resonances and to show precisely, how they build up within femtoseconds.” “These quantum effects are so fast that on our usual time scales they appear to happen instantaneously, from one moment to the next,” says Stefan Nagele. “Only by employing new sophisticated methods of attosecond physics it has become possible to study the time evolution of these processes.”
This does not only help quantum scientists to understand quantum effects, it also opens up new possibilities of controlling such processes – for example facilitating or inhibiting chemical reactions. https://www.tuwien.ac.at/en/news/news_detail/article/124462/
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