Core level spectroscopy
While an excitation of valence band electrons may already by achieved by
the light of gas discharge lamps, only synchrotron radiation sources offer a
sufficient photon energy for the photoionizaton of subshell states. The
application of soft x-ray radiation" (200 eV ≤ hν ≤ 700 eV)
for a direct photoemission process allows the energy- and angle-resolved
detection of core level electrons. In analogy to valence band photoemission
techniques x-ray photoelectron spectroscopy (XPS) uses the kinetic energy
of the photoexcited electrons to derive the binding energy of the initial
electronic state which is directly related to the ionization energy of the
appropriate atomic orbital. The measured photoelectron intensity
I(Ekin,hν,ϑ,φ) may therefore be
evaluated (i) for a determination of the chemical composition, (ii) for an
analysis of the atomic binding conditions, and (iii) for a depth-profiling
of the sample constituents in the surface-near region.
(i) Due to its occupied atomic orbitals each atom exhibits different
ionizable energy levels, which are reflected as element-specific peaks in
the x-ray photoelectron spectrum. As these emissions, which are
characterized by their binding energies, simply overlay each other in
compounds or mixtures, ESCA (Electron Spectroscopy for Chemical
Analysis) measurements allow the identification of all participating
elements by determination of their atomic core level lines. If additional
information concerning the cross section of the electronic transition, the
mean free path of the electrons and the spectrometer efficiency is provided,
even a determination of the chemical stoichiometry is possible [1].
(ii) The exact peak position of core level emissions in XPS spectra are
governed by the oxidation level of the emitting atom and the electric
field generated by adjacent atoms [2]. Generally, in case of a
coordination with more electronegative ligands the core level electrons of
a central atom appear at higher binding energy making them more
difficult to excite. This so-called chemical shift may influence the
energetic position of the emission peak up to 10 eV [1], which
enables an identification of binding partners and a distinction of single or
double covalent bonds. Therefore, high-resolution x-ray photoelectron
spectroscopy is the preferred technique to trace chemical reactions or
redox processes.
(iii) Keeping the sample orientation to the light source unchanged a
variation of the detection angle ϑ in the XPS environment allows
to tune the averaged information depth z ∝ cosϑ of the
emitted photoelectrons. Using the analyzer angle ϑ as a parameter
the peak ratio of elemental core level emissions mirrors an eventual
concentration gradient perpendicular to the surface. From the statistical
error and from the number of angular-dependent spectra a depth-resolved
profile of the atomic distribution may be derived with defined accuracy
[3]. A suitable electron energy ensures a sufficient
variation of the escape depth for different emission angles, so that a
distinction between intercalates and adsorbates becomes possible. In summary these application options ensure the convenience of x-ray photoelectron spectroscopy for the chemical analysis of solid state surfaces.
References
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