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X-ray photoelectron spectroscopy (XPS) is a powerful technique to investigate the chemical composition and the electronic structure of a large variety of materials, ranging from metals, semiconductors, insulators and superconductors to carbon-based materials, such as organic semiconductors.

The information depth of XPS is determined by the inelastic mean free path (IMFP) of the photo-excited electrons in solid matter. The Inelastic Mean Free Path as a function of the kinetic energy shows a distinct minimum at kinetic electron energies between 40–100 eV. The maximum kinetic energy of photoelectrons in an XPS experiment is defined by the photon energy. Here, typical photon energies used at synchrotron radiation facilities and in laboratories are up to 1500 eV. For such experiments the IMFP results in an information depth of 10–25Å. In other words, conventional XPS is a surface sensitive technique. To gain access to bulk and interface properties,the kinetic energy of electrons has to be increased by using higher photon energies for excitation. In hard X-ray photoelectron spectroscopy (HAXPES) photon energies typically range between 3 keV and 15 keV, which extends the information depth to 100–200 Å. Due to the low photoionization cross sections at higher excitation energies, special considerations have to be made regarding electron detection. Low dark-count detector units with linear response and high dynamic range, as well as high stability power supplies are needed. Furthermore the analyzer lens must work with high transmissionat high retarding ratios to provide a high energyresolution within the hard X-ray energy range.


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