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DS: Fachverband Dünne Schichten

DS 28: [O] Plasmonics and Nanooptics IV (Joint Session DS/O/HL)

DS 28.9: Vortrag

Mittwoch, 24. März 2010, 17:00–17:15, H32

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

We use the combination of a spectroscopic Photoemission Electron Microscope (PEEM) with femtosecond (fs) laser pulses to investigate samples with high spatial and temporal resolution. For proper imaging, the laser pulses must have sufficiently low intensity to minimize the space charge. Here, we report on experiments with a regenerative Ti:sapphire amplifier system. The laser setup generates fs-pulses (λ=800 nm, i.e., a photon energy of E=1.55 eV). The system has a variable repetition rate up to 250 kHz. 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=6.2 eV) of the amplifier system to evaluate space charge effects in PEEM. The space charge effect is reflected in an energetic broadening of the electron distribution and, at higher laser fluence, in a loss of focus. Insertion of apertures into the electron path provides a way to change the electron density in different areas of the microscope and locate in which section of the PEEM the space charge effect is dominant. The influence of the laser energy, wavelength, and repetition rate on the energy spectra and the image distortions will be discussed.