Study: Physicists create giant trilobite Rydberg molecules

Sketch of a trilobite Rydberg molecule. a Sketch of a Rydberg molecule. The coordinates of the Rydberg electron (blue) and ground state atom (green) relative to the Rydberg core (red) are indicated by black arrows. The relevant spins are those of the Rydberg electron s1the electron in the ground state atom s2 and the nuclear spin of the ground state atom I. b Sketch of a trilobite molecule. The Rydberg core and the ground state atom are shown (with exaggerated size) as red and green balls respectively. The electronic probability density plotted in 2D is shown by the density of blue dots. Credit: Communication in Nature (2023). DOI: 10.1038/s41467-023-43818-7

The Kaiserslautern physicist in the team of Professor Dr. Herwig Ott succeeded for the first time in directly observing the pure trilobite Rydberg molecule. Especially interesting is that these molecules have a unique shape, reminiscent of trilobite fossils. They also have the largest electric dipole moments of any molecule known so far.

The researchers used a dedicated device capable of preparing these fragile molecules at ultralow temperatures. The results reveal their chemical bonding mechanisms, which are different from all other chemical bonds. The study was published in the journal Communication in Nature.

For their experiment, the physicists used a cloud of rubidium atoms cooled in an ultra-high vacuum to about 100 microkelvin0.0001 degrees above absolute zero. After that, they excited some of these atoms into the so-called Rydberg state using lasers. “In this process, the outermost electron in each case is carried into distant orbits around the atomic body,” explains Professor Herwig Ott, who researches ultracold quantum gases and quantum atom optics at the University of Kaiserslautern-Landau. .

“The orbital radius of the electron can be more than a micrometer, making the electron cloud larger than a small bacterium.” Such highly excited atoms are also formed in interstellar space and are highly chemically reactive.

When a ground state atom is now located inside this giant Rydberg atom, a molecule is formed. While standard chemical bonds are covalent, ionic, metallic or dipolar in nature, trilobite molecules are bound by a completely different mechanism.

“This is the quantum mechanical scattering of the Rydberg electron from the ground state atom, which binds the two together,” said Max Althn, who is the first author of the study. “Imagine an electron rapidly orbiting around the nucleus. On each round trip, it collides with the ground state atom. Contrary to our intuition, quantum mechanics teaches us that these collisions lead to an effective attraction between of the electron and the ground state atom.”

The properties of these molecules are unique: Due to the wave nature of the electron, multiple collisions lead to an interference pattern that looks like a trilobite. In addition, the bond length of the molecule is as large as the Rydberg orbit which is larger than that of any diatomic molecule. And because the electron is very strongly attracted to the ground state atom, the permanent electric dipole moment is very large: more than 1,700 Debye.

To observe these molecules, scientists have built a dedicated vacuum apparatus. This allows the preparation of ultracold atoms by laser cooling and the subsequent spectroscopic detection of molecules. The results will help us understand the fundamental bonding mechanisms between ground state atoms and Rydberg atoms, which has recently become a promising platform for quantum computing applications. The researchers’ discovery has increased the understanding of Rydberg systems, which can be exotic and useful at the same time.

More information:
Max Althn et al, Exploring the vibrational series of pure trilobite Rydberg molecules, Communication in Nature (2023). DOI: 10.1038/s41467-023-43818-7

Provided by Rheinland-Pflzische Technische Universitt Kaiserslautern-Landau

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