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

[1]
C. Wagner, W. Riggs, L. Davis, and J. Moulder, in Handbook of x-ray photoelectron spectroscopy, edited by G.E. Muilenberg (Perkin Elmer Corporation, Eden Prairie, Minnesota, 1979).

[2]
P. Bagus, F. Illas, G. Paccioni, and F. Parmigiani, J. Electron. Spectrosc. relat. Phenom. 100, 215 (1999).

[3]
P. Cumpson, Appl. Surf. Sci 144-145, 16 (1999).