Table of Contents:
In this work the electron dynamics at the PTCDA/Ag(111) interface have been studied with time- and angleresolved Two-photon photoemission (2PPE) as well as time-resolved photoluminescence (PL).
The first part of this work concentrates on the characterization of an unoccupied electronic state, that develops 0.6 eV above the Fermi level due to the adsorption of the PTCDA molecules, whereas the shockley surface state of the clean surface vanishes. The measurements clearly identify this state as an interface state that is located between the metal surface and the first layer of the molecules. Dispersion measurements yield an effective mass of this state of 0.39 m_e at the Gamma-point and show backfolding at the zone boundaries of the rectangular PTCDA unit cell. Time-resolved measurements show a surprisingly short lifetime of t=54 fs, clearly indicating a strong coupling of the state with the metal. This behaviour can be explained by a shift of the shockley surface state. This for the clean Ag(111)-surface normally occupied state shifts above the Fermi level because of the highly polarizable PTCDA molecules. Calculations with a one dimensional model potential support this interpretation. Angleresolved lifetime measurements as a function of paralell momentum show a corelation of the decay dynamics of the interaface state with the measured bandstructure. The observed drop of the lifetime for larger paralell momentum is significantly smaler as expected for the pure shockley state. This behaviour can be explained due to a hybridsation of the shockley state with the LUMO+1 of the first PTCDA monolayer for k||>>0.
The second part of this work deals with the intramolecular excitation at the PTCDA/Ag(111) interface after excitation with laser pulses with 2.33 eV and 4.66 eV photon energy. Time-resolved photoluminescence measurements show a strong rise in the PL-lifetime as a function of PTCDA coverage, that can be explained by an increase in the crystallinity of the PTCDA films. After excitation with 4.66 eV + 4.66 eV the LUMO, LUMO+1 and the HOMO of the PTCDA multilayers can be identified by varying of the photon energy. The LUMO exhibits a stronger binding after resonant excitation with 2.33 eV, that is associated with the creation of an exciton. Time-resolved 2PPE measurements show a very long lifetime above the Fermi level that displays a multiexponentiel decay. Systematic measurements as a function of PTCDA coverage at 300 K and 85 K show a strong increase in the lifetime with a saturation above 100 ML, that can be clearly correlated to an intramolecular excitation. A Model in which the excited electrons in the PTCDA layers can get to metal surface by hopping processes is able to give a good qualitative description of the complex 2PPE-signal.