Valence region bulk solids

The low BE region of XPS spectra (<20 — 30 eV) represents delocalized electronic states involved in bonding interactions [7]. Although UV radiation interacts more strongly (greater cross-section because of the similarity of its energy with the ionization threshold) with these states to produce photoelectrons, the valence band spectra measured by ultraviolet photoelectron spectroscopy (UPS) can be complicated to interpret [1], Moreover, there has always been the concern that valence band spectra obtained from UPS are not representative of the bulk solid because it is believed that low KE photoelectrons have a short IMFP compared to high KE photoelectrons and are therefore more surface-sensitive [1], Despite their weaker intensities, valence band spectra are often obtained by XPS instead of UPS because they provide... 

When synchrotron radiation became available in the seventies. X-ray absorption spectroscopy took a jump in the volume of data taken and in their precision. These sources made absorption measurements in the soft X-ray region (M absorption in the lanthanides) and in the X-ray UV (N absorption in the lanthanides) much more easily accessible. At the same time, interest in information about solid state physics and chemistry of the lanthanides intensified with the discovery of valence fluctuations or homogeneously mixed valence. Accordingly the bulk part of this chapter is dedicated to a review of modern X-ray absorption spectroscopy of lanthanides, in particular of mixed valent materials, using synchrotron radiation. 

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