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X-ray Emission Spectroscopy

Figure 1
X-ray emission spectroscopy (XES) is a classical technique to study the electronic structure of bulk samples, for which it is ideally suited due to the large information depth (roughly 0.1 μm for soft X-rays). This technique can be made surface sensitive trough a selective, resonant excitation of a core-hole state at the atomic center of an adsorbate. Due to the localization of the core-hole, the local electronic structure of an atom in the adsorbate is probed selectively and no contributions come from the much larger number of atoms in the substrate. The emitted radiation is dominated by the decay of valence electrons the same atomic center. XES therefore probes the occupied valence states in a atom-specific projection. This is schematically shown in figure 1 for N2 adsorbed on a nickel surface. From the total charge density (gray envelope), valence electrons with p-angular momentum (contour lines) decay into N 1s core-holes and the total density of states is projected on its N 2p components. By selective excitation of only one nitrogen atom, the electronic structure can be either projected on the inner or the outer nitrogen atom.
Figure 2
In case of highly oriented systems angular dependent XES measurements yield information on the symmetry of the involved orbitals. This is illustrated in figure 2 for CO adsorbed on a metal surface. The maximum X-ray emission is generally found in the direction perpendicular to the spatial orientation the involved atomic p orbitals. By switching the direction of detection from normal to grazing emission orbitals of different spatial orientation are probed. In normal emission geometry, only valence states of π-symmetry contribute to the XE signal, whereas in grazing emission geometry both π and σ orbitals are probed. A simple subtraction procedure reveals σ states only.
Since X-ray fluorescence spectroscopy yields only about one photon emitted per 1000 photons the experiment requires a high intensity X-ray beams provided by a 3rd generation synchrotron.
Stockholm University |AlbaNova University Center| Fysikum |Updated 14 June 2010 | back to top |