Photo-electron Emission Spectroscopy and MicroscopyCopyright: Forschungszentrum Jülich | Daniel Wilson
Photoemission spectroscopy (PES) and photoemission electron microscopy (PEEM) are powerful tools for studies of electronic structure, magnetic properties, composition and morphology of matter. A crucial component in the experiment is the light source itself. Depending on the spectral region, photoemission spectroscopy can be roughly divided in ultraviolet electron spectroscopy (UPS) and X-ray electron spectroscopy (XPS).
For XPS and related techniques, like near edge x-ray absorption fine structure (NEXAFS) measurements, synchrotrons and free electron lasers with their spectral range from hard x-rays to extreme ultraviolet are unprecedented and unrivalled light sources in terms of available range and variability of photon energies, intensity, bandwidth, polarization and pulse duration.
For laboratory-based photoemission studies the various available light sources can be classified according to their photon energy. Most of the light sources operate at photon energies between 3 eV and about 40 eV. For XPS, X-ray tubes emitting e.g. Al Kα radiation (1 488 eV) are often employed. The classical light source for valence band spectroscopy, a He discharge lamp, emits two prominent lines at 21.2 eV (He I) and 40.8 eV (He II) . Sometimes also radiation from visible and ultraviolet lasers (photon energy > 3 eV) is used for photoemission studies of electronic states very close to the Fermi energy as well as unoccupied states (two- photon photoemission ). Such techniques often require additional sample treatment for reduction of the work function. A powerful tool for time-resolved studies of electronic states on the femtosecond timescale are laser-generated high harmonics. Successful high harmonics photoemission spectroscopy and microscopy experiments employed photon energies between 22 eV and 93 eV [3-8].
In this project, we combine a high power multi-kHz EUV light source and a state-of-the-art photoemission spectro-microscope. For this purpose, we design different monochromator geometries to select single, narrow-bandwidth spectral lines . The goal of the project is to extend the available photon energy range for laboratory-based photoemission spectroscopy and spectro-microscopy beyond the He II line and to close the spectral gap of laboratory light sources that currently exists between 40 eV and the X-ray spectral range.
The gas-discharge EUV light sources used in our projects were developed at the Fraunhofer Institute for Laser Technology in Aachen [9,10] and used in a variety of applications such as EUV lithography , reflectometry , magneto-optics  as well as EUV and soft X-ray microscopy [14-16]. EUV radiation is emitted from excited, multiply ionized gas atoms in a Z-pinch plasma. The spectral range of our gas-discharge EUV light sources extends from 25 eV to about 620 eV, depending on the electrical energy coupled into the plasma as well as the gas species. The extended spectral range and highly monochromatic spectral lines immediately open the door for core level spectro-microscopy and thus element- and chemical environment-specific studies which currently are only feasible at the synchrotrons and free electron lasers.
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|Dr.Denis Rudolf Dipl.-Phys.|
|+49 2461 6196469|
|Gordon Staab B.Sc.|
|+49 2461 6196466|
|Daniel Wilson M.Sc.|
|+49 2461 6196469|