Free-electron laser allows scientists to observe a molecular ball bearing

Tuesday, 3rd January 2017Miscellaneous
Graduate students Matias Fagiani (left) and Sreekanta Debnath (right) in front of the undulator of the free-electron laser at the Fritz Haber Institute.

Graduate students Matias Fagiani (left) and Sreekanta Debnath (right) in front of the undulator of the free-electron laser at the Fritz Haber Institute.

The free-electron laser (FEL) at the Fritz Haber Institute (FHI) generates intense pulses of infrared radiation of widely tunable wavelength. Unlike conventional lasers, where the radiation is produced in a gas, liquid, or solid, in an FEL it is generated by an electron beam propagating freely through a vacuum tube. In a device called undulator (Fig. 1),strong magnetic fields of alternating polarity force the electrons on a wiggling (undulating) motion, thereby causing the emission of radiation. The radiation wavelength can be tuned simply by varying the electron energy or the magnetic field strength. Before entering the undulator, however, the electrons need to be accelerated to almost the speed of light, requiring a complex electron accelerator. Since 2013 such an installation has been operational at the Fritz Haber Institute.

In collaboration with scientists from the Wilhelm Ostwald Institute for Physical und Theoretical Chemistry (Leipzig University) and from the Institute for Optics und Atomic Physics (Technical University Berlin) the FEL radiation has been applied to investigate a very special molecular system, the boron cluster B13+. It was known previously that 13 boron atoms can form a highly stable compound, referred to as a magic cluster. It is a planar system composed of two concentric rings; an inner B3-ring and an outer B10-ring (see Fig. 2). The key feature is that the system is very stable but not rigid. Previously, Thomas Heine (Theoretical Chemistry, Leipzig) and his coworkers predicted that it is possible for the rings to rotate like a ball bearing with no detrimental effect on the stability of the system. Pairs of electrons serve as the bearing’s balls, enabling the inner and outer rings to counter rotate effectively free of friction.titelbild_en_png

The group of Knut Asmis (Physical Chemistry, Leipzig), jointly with André Fielicke (TU Berlin), succeeded to prepare a single-isotope form of this very special boron system. And application of the FHI FEL radiation provided evidence for its concentric ring structure and also for the predicted rotational motion. The intense and tunable IR radiation from the FEL made it possible to measure the vibrational spectrum of B13+, revealing a fingerprint of the possible motions within the cluster. The spectrum exhibits clear evidence for the quasi rotation of the two rings.

Wieland Schöllkopf, who is heading the FHI FEL facility, points out that these results could not have been achieved with a conventional laser source. Hence, it represents another impressive example of possible applications of FEL radiation. Furthermore, demonstrating a molecular ball bearing consisting of just 13 atoms presents another example from the young and exciting research field of “Molecular Machines”, the development of which was just awarded by the 2016 Nobel Prize in Chemistry.


Structure and Fluxionality of B13+ Probed by Infrared Photodissociation Spectroscopy
M.R. Fagiani, X. Song, P. Petkov, S. Debnath, S. Gewinner, W. Schöllkopf, T. Heine, A. Fielicke, K.R. Asmis
Angew. Chem. Int. Ed., 56, 501–504, (2017)