A team of physicists at the Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau has produced pure trilobite molecules in rubidium over a wide range of frequencies and characterize their binding energies, lifetimes, and dipole moments.
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Sketch of a Rydberg molecule (a): the coordinates of the Rydberg electron (blue) and ground state atom (green) relative to the Rydberg core (red) are denoted with black arrows; the relevant spins are that of the Rydberg electron s1, the electron of the ground state atom s2 and the nuclear spin of the ground state atom I. Sketch of a trilobite molecule (b): the Rydberg core and the ground state atom are shown (with exaggerated size) as red and green spheres respectively; the electronic probability density projected to 2D is indicated by the density of blue dots. Image credit: Althön et al., doi: 10.1038/s41467-023-43818-7.
“Creating controllable molecules at ultralow temperatures offers a pathway to engineered ultracold quantum chemical reactions and tests of fundamental physics and symmetries,” said senior author Professor Herwig Ott and colleagues.
“Molecules that possess sizeable electric dipole moments can be controlled by external electric fields making them candidates for quantum information processing and the production of strongly correlated many-body systems.”
“For dipolar molecules with multiple vibrational states electric field pulses have been proposed to create superposition states and observe coherent wave-packet dynamics.”
“Ultralong-range Rydberg molecules are a platform for creating such dipolar molecules in ultracold environments.”
For their experiment, the physicists used a cloud of rubidium atoms that was cooled down in an ultra-high vacuum to about 100 microkelvin (0.0001 degrees above absolute zero).
Subsequently, they excited some of these atoms into a Rydberg state using lasers.
“In this process, the outermost electron in each case is brought into far-away orbits around the atomic body,” Professor Ott said.
“The orbital radius of the electron can be more than one micrometer, making the electron cloud larger than a small bacterium.”
“Such highly excited atoms are also formed in interstellar space and are chemically extremely reactive.”
“If a ground state atom is now located within this giant Rydberg atom, a molecule is formed.”
“While standard chemical bonds are either of covalent, ionic, metallic or dipolar nature, the trilobite molecules are bound by a completely different mechanism.”
“It is the quantum mechanical scattering of the Rydberg electron from the ground state atom, which sticks the two together,” said Dr. Max Althön, first author of the study.
“Imagine the electron rapidly orbiting around the nucleus. On each round trip, it collides with the ground state atom.”
“In contrast to our intuition, quantum mechanics teaches us that these collisions lead to an effective attraction between the electron and the ground state atom.”
“The properties of these molecules are amazing: due to the wave nature of the electron, the multiple collisions lead to an interference pattern which looks like a trilobite.”
“Moreover, the bond length of the molecule is as large as the Rydberg orbit — way bigger than any other diatomic molecule.”
“And because the electron is so strongly attracted by the ground state atom, the permanent electric dipole moment is extremely large: more than 1,700 Debye.”
The study was published in the journal Nature Communications.
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M. Althön et al. 2023. Exploring the vibrational series of pure trilobite Rydberg molecules. Nat Commun 14, 8108; doi: 10.1038/s41467-023-43818-7
