Free the Electron to Better Trap It

Researchers from UNIGE and MBI Berlin have for the first time placed an electron in a dual state - neither released nor bound - confirming a hypothesis of the 1970s.

electronic structure
Schematic representation of Kramers Henneberger's potential formed by the juxtaposition of the atomic potential and a powerful laser beam. © UNIGE - Xavier Ravinet

Atoms are formed of electrons around a central core to which they are linked. Electrons can also be torn-ionized-from their nucleus using the powerful electric field of a laser. In the 1970s, two theoreticians, Kramers and Henneberger, wondered whether it was possible to release the electron from its nucleus while keeping it captive to the laser beam. This hypothesis, considered impossible by many scientists, has been successfully confirmed by physicists from the University of Geneva (UNIGE) and the Max Born Institute (MBI) in Berlin. For the first time, they managed not only to control the shape of the laser pulse in order to keep the electron free of its nucleus, but also to regulate the electronic structure of this atom dressed by the laser. They also identified an area of ​​lawlessness, nicknamed “Valley of Death”, in which physicists lose all power over the electron. These results, to read in the journal Nature Physics , upset the theories and predictions concerning the ionization of matter.

Since the 1970s, several experiments have tried to confirm the hypothesis put forward by the theoreticians Kramers and Henneberger: we can place an electron in a double state, neither liberated nor bound. Trapped in the laser, the electron would be forced to pass and pass in front of its core and thus suffer the electric field of the laser combined with that of the nucleus. This dual state would control the electrons subjected to both the electric field of the nucleus and the laser and open the way for the creation of “new atoms“, from the point of view of their electronic structure, by physicists. But is it possible?

Act on the natural oscillations of the electron

Current theories state that the more intense the laser is, the easier it is to ionize the atom, that is to say to pull the electrons out of the electric field of their nucleus and to release them in space. “But once the ionized atom, the electrons leave not only the electric field of the nucleus of the atom, but also that of the laser,” says Jean-Pierre Wolf, professor at the Physics Section of the Faculty of Science of the University of Geneva. “Then we wanted to know if it was possible to trap them in the laser, once released from their core, as suggested by the hypothesis of Kramers and Henneberger,” he adds.

The only way to achieve this is to find the right shape of the laser pulse to apply, to impose the electron perfectly similar oscillations so that its energy and its state remain stable. “Indeed, the electron oscillates naturally in the field of the laser, but these oscillations are not regular and push the electron to change constantly its energy level and therefore its state, that is why it escapes from the electric field of the laser, “adds Mikhail Ivanov, professor at the MBI‘s Berlin Department of Theory.

Modulate the intensity of the laser to avoid the Valley of Death

Physicists have tested several laser intensities to obtain the regularity of the oscillations of the electron released from its nucleus. They then made a surprising discovery. “Contrary to current theories that suggest that the more intense the laser, the easier it is to ionize the electron, we have discovered that there is a limit of intensity where we can no longer ionize the atom,” says Mikhail Ivanov . Beyond this threshold, we find the possibility of curbing it. “The researchers named this term the” Valley of Death “, a proposal made by Professor Joe Eberly of the University of Rochester.

After several adjustments, the physicists of UNIGE and MBI managed for the first time to release the electron from its nucleus, then to trap it in the electric field of the laser, as suggested by Kramers and Henneberger. “By applying an intensity of one hundred trillion Watt per cm2, we were able to cross the threshold of Death Valley and trap the atom in a cycle of regular oscillations within the laser’s electric field,” enthuses Jean-Pierre Wolf. For comparison, the intensity of the Sun on the earth is of the order of 100 Watt per m2.

Confirm an old hypothesis that revolutionizes physical theory

By placing the electron in a dual state, neither bound nor released, the researchers have found a way to manipulate their oscillations as they wish, which allows them to act directly on the electrical structure of the atom. “This gives us the opportunity to create new atoms dressed by the laser field, with new energy levels of electrons,” says Wolf. “We thought that this double state was impossible to achieve and we have just proved the opposite. This will play a fundamental role in theories and predictions about the propagation of intense lasers, “he concludes.

Source : University of Geneva