Berlin 2008 – scientific programme

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O: Fachverband Oberflächenphysik

O 52: Time-Resolved Spectroscopy III

O 52.5: Talk

Wednesday, February 27, 2008, 16:15–16:30, MA 042

Reversible switching of azobenzene in direct contact with a Au(111) surface — •Sebastian Hagen, Peter Kate, Felix Leyssner, Martin Wolf, and Petra Tegeder — Freie Universität Berlin, Fachbereich Physik

Two-photon photoemission spectroscopy is employed to analyze reversible changes in the electronic structure of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111), which are induced by light and thermal activation. Cycles of illumination and annealing steps confirm the reversibility of the switching process, which we assign to a trans/cis-isomerization of TBA molecules in direct contact with the Au(111) surface. Based on wavelength dependent measurements we propose that the optically-induced conformational change (trans- to cis-isomerization) of the adsorbed molecules is not caused by direct intramolecular excitation as in the liquid phase but rather by an indirect mechanism, viz. a substrate mediated process. Thereby photoexcitation of holes in the Au d-band followed by a charge transfer to the HOMO level drives the isomerization in the photon energy range between 2.1 and 4.6 eV. In addition, for photon energies above 4.6 eV the creation of a negative ion resonance via attachment of excited electrons to the molecules plays a role.