Kaiserslautern physicists led by Professor Dr. Herwig Ott have succeeded in directly observing trilobite molecules for the first time. These very large molecules get their name from their resemblance to fossil trilobites. Due to their size, they have the largest electric dipole moments of any molecule known to date. The researchers have used a special apparatus with which these fragile molecules can be produced at extremely low temperatures. The results are important for understanding their chemical bonding mechanisms, which differ from all other chemical bonds. The study was published in the renowned journal "Nature Communications".
For their experiment, the physicists used a cloud of rubidium atoms that was cooled to around 100 microkelvins - 0.0001 degrees above absolute zero - in an ultra-high vacuum. They then used lasers to excite some of these atoms into a so-called Rydberg state. "The outermost electron is brought into distant orbits around the atomic nucleus," explains Professor Herwig Ott, who researches ultracold quantum gases and quantum atom optics at the Rhineland-Palatinate University of Technology Kaiserslautern-Landau (RPTU). "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 also form in interstellar space and are extremely chemically reactive.
If there is now another atom inside this huge Rydberg atom, a molecule is formed. While conventional chemical bonds are either covalent (bonding via a pair of electrons), ionic (bonding via positively and negatively charged ions), metallic (freely moving electrons) or dipolar in nature (bonding through dipole forces), the trilobite molecules are bonded by a completely different mechanism. "It is the quantum mechanical scattering of the Rydberg electron on the atom that glues the two together," says Max Althön, first author of the study. Althön continues: "Imagine the electron orbiting the atomic nucleus in a fast orbit. Each time it orbits, it collides with the second atom. Contrary to our intuition, quantum mechanics teaches us that these collisions result in an effective attraction between the electron and the atom."
These molecules have amazing properties: Due to the wave nature of the electron, the multiple collisions result in an interference pattern that looks like a fossil trilobite. In addition, the bond length of the molecule is as large as the Rydberg orbit - much larger than that of any other diatomic molecule. And because the electron is so strongly attracted to the second atom, the permanent electric dipole moment is extremely large at more than 1700 Debye.
In order to observe these molecules, the scientists have developed a special vacuum apparatus. It makes it possible to produce ultracold atoms by laser cooling and then detect the molecules spectroscopically. The results contribute to the understanding of fundamental bonding mechanisms between atoms in the ground state and Rydberg atoms, which have recently also become a platform for quantum computing applications. The researchers' discovery adds to the understanding of Rydberg systems, which can be both exotic and useful.
The work on this study took place as part of the priority program "Giant Interactions in Rydberg Systems", which is funded by the German Research Foundation. The research was carried out in the OPTIMAS profile area (State Research Center for Optics and Materials Science), which has been funded since 2008 as part of the research initiative of the state of Rhineland-Palatinate.
The results of the measurements and a description of the experimental setup have been published in the renowned journal Nature Communications: "Exploring the vibrational series of pure trilobite Rydberg molecules"; Max Althön, Markus Exner, Richard Blättner & Herwig Ott
www.nature.com/articles/s41467-023-43818-7
DOI: doi.org/10.1038/ s41467-023-43818-7
Questions answered:
Prof. Dr. Herwig Ott
Ultrakalte Quantengase und Quanten-Atom-Optik
RPTU in Kaiserslautern
E-Mail: herwig.ott@rptu.de
Telefon: +49 0631 205-2817