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X-ray Emission Spectroscopy
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| | 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.
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| | 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.
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