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Berlin 2018 – scientific programme

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O: Fachverband Oberflächenphysik

O 68: Surface dynamics: Reactions, elementary processes and phase transitions I

O 68.2: Talk

Wednesday, March 14, 2018, 15:15–15:30, MA 043

Analyzing and Tuning the Energetic Landscape of H2Pc Tautomerization — •Lucas Klein1, Jens Kügel1, Markus Leisegang1, and Matthias Bode1,21Physikalisches Institut, Experimentelle Physik II, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany — 2Wilhelm Conrad Röntgen Center of Complex Material Systems (RCCM), Universität Würzburg, Am Hubland, 97074 Würzburg

Many chemical and biological reactions are associated with the quantum tunneling of hydrogen atoms through a potential barrier in the reaction path [1]. Whereas quantum tunneling is often identified by temperature-dependent measurements or a large kinetic isotope effect, we demonstrate a new method working at a fixed temperature. As a model system we used the tautomerization—the switching of hydrogen protons between different adsorption sites—of phthalocyanine (H2Pc) molecules adsorbed on a Ag(111) surface which are probed by scanning tunneling microscopy. We will show, that the tautomerization of this molecule is exclusively triggered by the excitation of the N–H stretching mode. Interestingly, a step-like increase of the tautomerization rate is observed at a bias voltage that corresponds to the second harmonic of this vibrational mode, which we attribute to a crossover from quantum tunneling through the tautomerization barrier to an excitation over the barrier. This explanation is further supported by modifying the energetic landscape of the tautomerization of pristine H2Pc by single silver atom manipulation and/or deprotonation.
[1] M. Koch et. al., J. Am. Chem. Soc. 139, 12681-12687 (2017)

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