Dresden 2009 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 27: Poster Session I (Methods: Scanning probe techniques; Methods: Atomic and electronic structure; Methods: Molecular simulations and statistical mechanics; Oxides and Insulators: Clean surfaces; Oxides and Insulators: Adsorption; Oxides and Insulators: Epitaxy and growth; Semiconductor substrates: Clean surfaces; Semiconductor substrates: Epitaxy and growth; Semiconductor substrates: Adsorption; Nano- optics of metallic and semiconducting nanostructures; Electronic structure; Methods: Electronic structure theory; Methods: other (experimental); Methods: other (theory); Solutions on surfaces; Epitaxial Graphene; Surface oder interface magnetism; Phase transitions; Time-resolved spectroscopies)

O 27.114: Poster

Dienstag, 24. März 2009, 18:30–21:00, P2

Space Charge Effects in the Photoemission Electron Microscope — •Jan Göhre, Niemma M. Buckanie, Ping Zhou, Dietrich von der Linde, Michael Horn-von Hoegen, and Frank-J. Meyer zu Heringdorf — University Duisburg-Essen, Germany

The illumination of samples in a spectroscopic Photoemission Electron Microscope (PEEM) with ultrafast laser pulses combines temporal and spatial resolution. To study the deexcitation of excited electrons, the laser should have sufficiently low pulse energy in order to minimize the space charge. Since most metals have a work function of about 5eV, a high photon energy is needed for a one photon photoemission process (1PPE). We built a regenerative Ti:sapphire amplifier system which generates ultrafast fs-pulses (λ=800nm, corresponding energy of E=1.55eV). The system has a variable repetition rate up to 250kHz. It uses chirped pulse amplification (CPA) to generate laser pulses with a duration of 150fs and variable pulse energies, e.g. 1.2µJ at 250kHz. We studied Ag islands which have been grown (in-situ) by self-assembly on Si(111) surfaces. We used the fundamental and the 4th harmonic (E=6.2eV) of the amplifier system to study the origin of the space charge effect, i.e., whether the space charge effects are created at the sample surface or in the electron optics of the microscope (Boersch effect). The space charge effect is reflected in an energetic broadening of the electron distribution. Insertion of apertures into the way of the electrons changes the electron density in the electron optics. We report the influence of the laser energy, wavelength, and repetition rate on the energy spectra and the image distortions by space charge effects.