While direct photoemission spectroscopy offers an experimental approach to to the occupied electronic bands of a solid state, XANES (x-ray absorption near-edge spectroscopy) or NEXAFS (near-edge x-ray absorption fine structure) is a technique to characterize surfaces by evaluation of unoccupied electronic states. In contrast to inverse photoemission spectroscopy the experimental setup simply requires a monochromatically tunable light source and an electron energy analyzer so that XANES measurements can be performed at each synchrotron radiation source of sufficient energy. Due to the sharp transitions in molecular systems near-edge absorption spectroscopy is one of the preferred experimental techniques to study organic thin films.
The XANES principle is based on the determination of the x-ray absorption coefficient m depending on the photon energy hn at a fixed angle of illumination q. As the optical excitation of a core level electron requires the binding energy EB as a minimum photon energy, the transgression of this energy will coincide with an increased absorption coefficient. This leads to the formation of absorption edges, which may be indexed by their atomic subshells (K,L,M...). Beyond the absorption edge the intensity of a monochromatic x-ray passing through a medium of thickness d will follow the absorption law