Kaiserslautern trilobite molecules on the cover of Physical Review Letters

Researchers led by Professor Herwig Ott have succeeded in spectroscopically investigating exotic trilobite molecules with unprecedented precision. These ultra-long molecules made of rubidium atoms are characterised by extraordinary binding energies and strong electric dipole moments. The shape of the electronic wave functions resembles fossil trilobites, which is why these molecules were named after them. The experiments not only enable a better understanding of quantum mechanical bonding mechanisms, but also provide new insights into fundamental electron-atom scattering processes. The results were published in the renowned journal Physical Review Letters, where they grace the front page.

In their setup, the physicists use a cloud of ultra-cold rubidium-87 atoms, which were cooled to about 40 microkelvin using laser cooling. Subsequently, atoms were transferred to highly excited Rydberg states using a targeted three-photon process. ‘This causes the outermost electron to be brought into an orbit that can be several thousand atomic radii in size,’ explains Professor Ott. "If another atom is now located in this area, it can form a bond with the Rydberg electron through quantum mechanical scattering – a mechanism that has nothing in common with classical chemical bonds.

The trilobite molecules created in this way have particularly high binding energies due to this special bond. With principal quantum numbers n = 22 to 27, several series of such molecules were generated in the current study and their vibration spectra were measured precisely. Thanks to the high binding energy and spectral resolution (better than 0.01%), it was possible for the first time to determine the electron-atom scattering phase with unprecedented accuracy – and in an energy range that is not experimentally accessible for free electrons.

A newly developed functional method was used for the theoretical description. This avoids convergence problems in previous models and allows precise calculation of the vibration spectrum. ‘The agreement between theory and experiment is so precise that even the smallest deviations become visible,’ says Markus Exner, who led the experiments. Particularly noteworthy is the measurement of the largest permanent electric dipole moments to date, at nearly 3000 Debye, which enables a clear assignment of the observed vibrational states.

The work makes a significant contribution to a better understanding of molecular physics in the ultra-cold temperature range and complex electron-atom interactions. In addition, it could serve as a basis for future applications in quantum information and simulation.

The study was conducted as part of the German Research Foundation (DFG) Priority Programme ‘Giant Interactions in Rydberg Systems’ and in the OPTIMAS profile area (State Research Centre for Optics and Materials Science) at the University of Kaiserslautern-Landau.

The results were published in the journal Physical Review Letters:

‘High Precision Spectroscopy of Trilobite Rydberg Molecules’; Markus Exner, Rohan Srikumar, Richard Blättner, Matthew T. Eiles, Peter Schmelcher & Herwig Ott.