Berlin 2008 – scientific programme
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O: Fachverband Oberflächenphysik
O 52: Time-Resolved Spectroscopy III
O 52.6: Talk
Wednesday, February 27, 2008, 16:30–16:45, MA 042
Electron dynamics at the PTCDA/Ag(111) interface studied with 2PPE — •Manuel Marks1, Christian Schwalb1, Sönke Sachs2, Achim Schöll2, Eberhard Umbach2,3, and Ulrich Höfer1 — 1Fachbereich Physik und Zentrum für Materialwissenschaften, Philipps-Universität Marburg, D-35032 Marburg — 2Universität Würzburg, Experimentelle Physik II, D-97074 Würzburg — 3Forschungszentrum Karlsruhe, D-76021 Karlsruhe
We investigated epitaxial grown PTCDA (3,4,9,10-perylene-tetracarboxylic acid-dianhydride) on the Ag(111) surface as model system for a metal-organic interface by means of time- and angle-resolved two-photon photoemission (2PPE). In the presence of thin PTCDA films, an unoccupied state with an effective electron mass of 0.39 me is observed in the projected band gap of Ag 0.6 eV above EF. Its inelastic electronic lifetime is ≃50 fs and the state has an appreciable metallic character, significantly exceeding that of the image-potential states. We assign the new state to a mixture of the former Ag(111) Shockley surface state and the LUMO+1 of the first PTCDA monolayer (ML). In contrast to this interface state, which changes only weakly with PTCDA coverage, the binding energy of the first image-potential state shows a strong dependence. It increases by 135 meV for 1 ML, compared to clean Ag(111), but with absorption of the second ML, a subsequent drop of −70 meV relative to the clean surface occurs. A similar coverage dependence can be seen in the effective electron mass, which decreases by 20% from the first to the second PTCDA layer.