DEPARTMENT OF
PHYSICAL CHEMISTRY
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Tip-enhanced Raman scattering (TERS) combined with highly stable scanning tunneling microscopy is a powerful tool to obtain local chemical information at surfaces with exceptional high spatial resolution. Here, we investigate the evolution of TERS at Angstrom-scale distances between tip and surface up to the point where tip apex touches the surface. A stable atomic point contact is reversibly formed in the junction between our plasmonic Ag tip and ultrathin ZnO films on a Ag(111) surface. A dramatic enhancement and abrupt increase of the TERS intensity occurs upon contact formation for ZnO, which attributed to strong hybridization between the Ag tip and ZnO enhancing the Raman polarizability. This is further corroborated by the appearance of a new vibrational mode arising from the atomic contact. In contrast, related control experiments on chemically inert NaCl films show no such enhancement and the absence of the point contact mode.

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The Gaede Prize 2020 is awarded to Dr. Takashi Kumagai for his outstanding work on near-field physics and chemistry in plasmonic nano-junctions. The Gaede award recognizes excellent work in the area of vacuum science and is awarded by the German Physical Society (DPG) in cooperation with the Gaede foundation to scientists in an early career stage.
Takashi Kumagai studied chemistry at the University of Kyoto and received his PhD degree in 2011 on visualization of hydrogen bond dynamics at surfaces. He subsequently joined our department as a postdoc with Leonhard Grill and is heading the “Nanoscale Surface Chemistry” group since 2013. He performed fundamental studies on tautomerization reactions on single molecules and developed new instrumentation in in scanning tunneling microscopy (STM) to address fundamental questions in nanoscale light–matter interactions using highly controlled plasmonic STM junctions.

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The Alexander von Humbaldt Foundation has announced that Prof. Dr. Jens Biegert, a researcher at ICFO, Castelldefels, has been granted the Friedrich Wilhelm Bessel Research Award. Professor Biegert is an international leader in the fields of ultrafast laser physics and attosecond science, and noted as a pioneer atomic- and attosecond-resolution molecular imaging, and key advances towards element-selective attosecond x-ray spectroscopy of carrier dynamics in solid state systems. During his stay in Berlin at the Fritz-Haber-Institut der Max-Planck-Gesellschaft he will explore the connection between electrons and the lattice within electronic and structural phase transitions and for superconductivity, Here he will be hosted by Prof. Dr. Martin Wolf.

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Physical properties often boil down to the arrangement of electrons within a solid, which means that mapping the electronic structure is a powerful way of classifying and understanding a wide range of materials. By applying ultrashort laser pulses with 22 eV photon energy from a novel 500 kHz extreme-ultraviolet light source to chains of indium atoms on a silicon surface, we are able to produce a detailed map of electronic states not normally accessible to traditional methods for measuring the electronic structure. The same technique also enables us to track the energy flow within the material as the highly excited electrons exchange energy with each other and the crystal lattice. Charting such previously inaccessible states and their dynamics allows us to better test the limits of current theories for understanding and predicting physical phenomena, and may in future lead to new designer materials.

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This publication by Shuyi Liu, Martin Wolf, and Takashi Kumagai in Physical Review Lettersreports about plasmon-assisted resonant electron tunneling from a silver or gold tip to field emission resonances (FERs) of a Ag(111) surface induced by continuous wave (cw) laser excitation of a scanning tunneling microscope (STM) junction at visible wavelengths. As a hallmark of the plasmon-assisted resonant tunneling, a downshift of the first peak in the FER spectra by a fixed amount equal to the incident photon energy is observed. STM-induced luminescence measurement for the silver and gold tip reveals the clear correlation between the laser-induced change in the FER spectra and the plasmonic properties of the junction. These results clarify a novel resonant electron transfer mechanism in a plasmonic nanocavity.

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Watching the motions of atoms in the course of a chemical reaction is generally thought of as the Holy Grail for understanding chemical transformations or phase transitions in solids. While recordings of such “molecular movies” have been achieved in recent years, the atomic motion does not reveal the whole story of why specific bonds break and others form. This is dictated by the arrangement of the electrons as the atoms move along gradients on an energy landscape defined by the electrons. It is therefore necessary to observe the dynamics of the electronic structure, which means to record an “electron movie”, to obtain a complete picture of the mechanisms driving chemical reactions.

An experimental team at the Fritz-Haber-Institut in Berlin and computational scientists at the University of Paderborn now filmed the electrons during a light-induced reaction. They investigated a single layer of indium atoms on top of a silicon crystal. At low temperatures, the indium atoms form an insulating layer with the atoms arranged as hexagons. At room temperature, however, the indium atoms rearrange and form conducting atomic wires. This phase transition can not only be induced by changing the temperature but also by exciting the cold material with a very short flash of light. This light pulse puts energy in the electrons of the material faster than the atoms can move. Due to the extra energy, the electrons reorganize and change the energy landscape for the atoms: the atoms immediately start to move. In turn, the swift electrons react to the change in the atomic structure. This dynamic interplay between electrons and atoms has been recorded with time- and angle-resolved photoemission spectroscopy: a second ultrashort laser pulse is used to emit few of the electrons at different times after the phase transition was initiated by the first laser pulses. By repeating this process billions of time, a movie of the electronic structure during the phase transition of the indium nanowires was obtained. This information, combined with simulations of the electronic structure dynamics, made it possible to translate the electronic structure dynamics into a movie of the atomic energy landscape. This detailed reconstruction of the reaction pathway reveals not only the motion of atoms but also the formation and breaking of chemical bonds during the phase transition.

The approach demonstrated by Nicholson et al. is generally applicable to physical processes like structural phase transitions in solids as well as to chemical reactions, for instance of molecules. The theoretical framework for describing the electronic structure, however, differ significantly between these cases: while electrons in a crystal are described as bands in momentum space, electrons in molecules are depicted as bonds in real space. The work by Nicholson et al. provides a bridge between the languages of physics and chemistry for describing photo-induced reactions. Understanding how the transient electronic structure results in bond dynamics may in future allow the tailoring of chemical reactions and phase transitions via engineered light pulses.