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Zusammenfassung:
Two-photon photoemission (2PPE) spectroscopy is employed to quantify the photochemically and thermally induced trans ⇌ cis isomerization of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on an Au(111) surface. The isomerization of TBA is accompanied by significant changes in the electronic structure, namely different energetic positions of the lowest unoccupied molecular orbital of both isomers and the appearance of an unoccupied final state for cis-TBA. A quantitative analysis of these effects allows the calculation of cross sections for the reversible isomerization and determination of the ratio between both isomers in the photostationary state, where 55±5% of the molecules are switched to cis-TBA. The cross section for the photoinduced trans → cis isomerization is 3.3±0.5×10−22 cm2, while for the back reaction, a value of 2.7±0.5×10−22 cm2 is obtained. Furthermore a pronounced reduction of the activation energy by a factor of four compared to the free molecule is found for the thermally activated cis → trans isomerization of the surface-adsorbed TBA. This demonstrates that the potential energy landscape of the adsorbed TBA is remarkably different from the liquid phase.
