Physik: DPG Tagungen - Sitzung O 28 - Nano-science.de

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Physik: DPG Tagungen - Sitzung O 28 http://www.dpg-tagungen.de/archive/2004/o_28.html their binding energies with respect to the Ar conduction band and also results in longer lifetimes. The dispersion of the states is found to be parabolic near the band minimum with an effective mass of 0.5 m e . O 28.71 Poster Mi 16:00 Bereich C Zeit- und energieaufgelöste Untersuchungen des Transports optisch angeregter Elektronen in Metall-Isolator-Metall-Kontakten •Sven Schramm 1 , Johannes Schneider 1 , Walter Pfeiffer 1 und Detlef Diesing 2 1 Physikalisches Institut Universität Würzburg, Experimentelle Physik 1, 97074 Würzburg 2 Institut für Schichten und Grenzflächen, Forschungszentrum Jülich, 52425 Jülich Metall-Isolator-Metallkontakte (MIM) bestehen aus zwei Metallelektroden die durch eine dünne Oxidschicht getrennt sind (Ta-TaO-Ag, Al-Al 2 O 3 -Ag). Sie bilden ein Modellsystem zur Untersuchung des lichtinduzierten Elektronentransports durch die Oxid-Barriere. Aufgrund der starken Zunahme der Transmission der Tunnelbarriere für optisch angeregte Ladungsträger werden diese nach Anregung mit ultrakurzen Lichtimpulsen als kurzer Elektronenpuls in die Gegenelektrode injiziert. Aufgrund der inhomogenen Anregung kommt es zu einem resultierenden Photostrom. Ausgehend von den bislang durchgeführten Photostrommessungen [1] wird dieser Injektionsvorgang nun auch energieaufgelöst untersucht. Hierzu werden in der Rückelektrode des Kontaktes Elektronen mit einem ultrakurzen Lichtimpuls angeregt und dann durch zeitkorrelierte Photoemission in der Deckelektrode nachgewiesen. Der Nachweis des Injektionstromes hängt stark von den Anregungsbedingungen und den Barriereneigenschaften ab. [1] Thon et al., Appl.Phys. A 78 (2004) 189 O 28.72 Poster Mi 16:00 Bereich C Time development of electronic excitations of CO on MgO(001) •N.-P. Wang 1 , M. Rohlfing 2 , P. Krüger 1 und J. Pollmann 1 1 Institut für Festkörpertheorie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster 2 School of Engineering and Science, International University Bremen, P.O. Box 750761, 28725 Bremen Femtosecond laser excitation is a very useful tool for acquiring important knowledge about the short-time dynamics of surface excitations. In this context the fundamental question arises how fast an initially localized excitation, which is not an eigenstate of the surface system but rather defines the 'initial value' of the time-dependent Schrödinger equation, develops in space and time. Here the dependence of the time decay on the initial molecular excitation and the electronic spectrum of the substrate surface is of particular interest. We have studied therefore the dynamics of electronic excitations in a CO adlayer on MgO(001)-(1×1) from first principles. One-particle excitations of the ground state, described within DFT-LDA, are calculated employing the GW approximation. Two-particle excitations and their optical spectrum are obtained solving the Bethe-Salpeter equation including electron-hole interaction. The time evolution of the molecular excitation is then determined from the time-dependent Schrödinger equation. An initial CO exciton state is found to decay exponentially within two femto-seconds after coupling the adlayer to the substrate. This is due to a resonance with charge-transfer excitations between the substrate and the adlayer. Interestingly, the decay of the molecular exciton is an order of magnitude faster than the decay of single-particle states. O 28.73 Poster Mi 16:00 Bereich C Ultrafast energy relaxation and transport effects in photoexcited metals and metallic thin films •Panagiotis Loukakos, Martin Lisowski, Uwe Bovensiepen, Julia Stähler, Cornelius Gahl und Martin Wolf Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin Time-resolved two-photon-photoemission spectroscopy is used to study the femtosecond dynamics of 26 of 32 07.06.2009 22:14
Physik: DPG Tagungen - Sitzung O 28 http://www.dpg-tagungen.de/archive/2004/o_28.html photoexcited electrons in bulk Ru(001), Cu(111) and thin Cu(111) films epitaxially grown on Si(111)-7x7. After excitation with photon energies of 1.5-2.4 eV the time evolution of the non-equilibrium electron population in the vicinity of the Fermi level is probed by a time-delayed UV pulse. For Cu film thicknesses between 6 nm to 44 nm we find that the electron relaxation rates increase monotonically with film thickness, which is attributed to electron transport out of the probe depth into the bulk. For the electron dynamics at the Ru(001) surface we observe an unexpectedly fast energy dissipation resulting in a decreased temperature at the surface with respect to predictions of the two-temperature model. We attribute this to the influence of the non-thermal electron distribution. With the aid of numerical calculations we propose an increased (ballistic) transport of non-thermalized electrons and a decreased electron-phonon coupling at the surface to account for the efficient energy dissipation into the bulk. O 28.74 Poster Mi 16:00 Bereich C Space Charge Effects during Ultrashort Laser Pulse Photoemission Electron Microscopy •D. Thien, M. Horn-von Hoegen, O. Heinz, P. Zhou, D. von der Linde und F.-J. Meyer zu Heringdorf Institut für Laser und Plasmaphysik, Universität Duisburg-Essen (Campus Essen), 45117 Essen Visualization of the dynamics of electronic states in mesoscopic structures requires the simultaneous application of 3 different tools: Ultrashort fs-laser pulses offer time resolution, spectroscopy allows separation of different excited electronic states and Photoemission Electron Microscopy (PEEM) provides the crucial possibility to study surface areas on the nanoscale. We are currently combining an amplified Ti:Sapphire laser system (40fs pulses at 1kHz repetition rate), and a PEEM with imaging energy filter (ELMITEC) into a novel pump-probe experiment. While our laser oscillator (20fs, 80MHz) typically images excited electronic states via 2 Photon Photoemission, the amplifier system provides a greatly improved intensity of more than 10 15 W/cm 2 and opens the door for frequency conversion and photoemission experiments with photon energies of more than 30eV. The intense ultrashort pulses confine a great number of photoemitted electrons into a small area resulting in coulomb interaction. Here we analyze these space-charge effects using frequency doubled laser pulses on patterned in-situ grown low-workfunction Cs-layers on Si and SiO 2 substrates. At high photon flux we observe that the edges of our Cs-pads appear blurred. Subsequent reduction of the laser peak power leads to sharper edges as are also observed when the 80MHz oscillator is used for illumination. O 28.75 Poster Mi 16:00 Bereich C Time-of-Flight Two-Photon Photoemission Spectromicroscopy of Noble metal cluster films with Femtosecond Laser Radiation. •M. Cinchetti, A. Gloskovskii, D. A. Valdaitsev, S. A. Nepijko und G. Schönhense Johannes Gutenberg-Universität, Institut für Physik, 55099 Mainz, Deutschland Time-of-Flight Two-Photon Photoemission Spectromicroscopy [1] was used to investigate the photoemission properties of some Nobel metal surfaces under femtosecond laser irradiation (photon energy 3.1 eV, pulse width < 200 fs, laser fluence 6.4 µJcm -2 ).In particular, we present the results of investigations of Cu surface inhomogeneities and Ag nanoparticle films deposited on a Si(111) substrate. The photoemission yield resulted to be enhanced in presence of surface inhomogeneities with dimensions in the nanometer range. The energy distribution curves obtained from regions characterized by a strongly enhanced photoemission yield are shown to have the same qualitative behaviour. They differ significantly from the energy distribution curves from the homogeneous and clean metal surfaces. The differences are explained in terms of (i) a reduction of surface potential barrier connected with the appearance of local fields between the nanoparticles and the substrate; (ii) the excitation of Localized Surface Plasmons in the nanoparticles and the consequent modification of the near field [2], that influences the photoemission. [1] M. Cinchetti, A. Oelsner, G. H. Fecher, H. J. Elmers, and G.Schönhense, Appl. Phys. Lett. 83, 1503 (2003). [2] V.M. Shalaev, C. Douketis, T. Haslett, T. Stuckless, and M. Moskovitis, Phys. Rev. B 53, 11193 (1996). O 28.76 Poster Mi 16:00 Bereich C 27 of 32 07.06.2009 22:14
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