Dresden 2006 – scientific programme

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O: Oberflächenphysik

O 32: Adsorption IV

O 32.5: Talk

Thursday, March 30, 2006, 12:15–12:30, TRE Phys

Conformational changes in adsorbed molecules — •Philipp Martin Schmidt, Thorsten U. Kampen, J. Hugo Dil, and Karsten Horn — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

Stilbene is the monomer building block of phenylenevinylene-type oligomers and polymers. It undergoes an interconversion around the central C=C double bond upon irradiation with UV-light of 250-320nm wavelength. This cis-trans-isomerization of the free molecule follows an in-plane hula-twist mechanism whose pathway can be assumed to work also in constraint systems as on surfaces - thus assessing a ’molecular switch’. To investigate the isomerization of stilbene-molecules on surfaces, planar trans(t)-stilbene has been studied on Si(100) surfaces at ~90 K by means of UPS and NEXAFS. Valence band spectra show that t-stilbene adsorbs undissociated onto the Si(100) surface. Neither Si2p nor C1s core level spectra show an energy shift upon coverage of the surface with t-stilbene, suggesting that the interaction is weak. Intensity changes of different components of the Si2p and C1s reveal that bonding takes place between Si dimers and the carbon atoms in the central C=C double bond. NEXAFS spectra indicate that the first layer of t-stilbene adsorbs in a flat orientation with the benzene rings being parallel to the substrate surface. Multilayers, on the other hand, physisorb highly disordered. A comparison of valence band spectra of trans- with cis-stilbene reveal significant differences which may permit an identification of both isomers for a further investigation of the isomerization process.