X-ray Photoelectron Spectroscopy

Photoelectron Spectroscopy using excitation energies greater than 100 eV is known as X-ray photoelectron spectroscopy (XPS). Typical excitation energies are at 1486.6 eV (characteristic X-ray line of aluminum, hereafter referred to as Al Kα) or at 1253.6 eV (Mg Kα line).
Due to the high excitation energy in the case of XPS mainly electrons from energetically lower shells are released, i.e. the observed electrons are from core. Therefore XPS is often referred to as "core level spectroscopy", in contrast to UPS where the excitation energies are below 100 eV, and the observed electrons are from the valence band.

All photoelectron spectroscopy is based on the photoelectric effect which was discovered and described by Einstein in 1905. By the irradiation with energy in the form of e.g. light electrons are released. Furthermore, these electrons have a defined kinetic energy associated with its binding energy and work function. This is given by equation 1.1.

(1.1)

With an energy-dispersive element the released electrons can be detected as a function of their kinetic energy.

Parallel to the X-ray induced release of photoelectrons a further process takes place, the Auger process. Electrons from a higher shell fall in the hole left by the photoelectron. In this process the excess energy is transferred to another electron which can then leave the atom (see Auger electron spectroscopy).

Since the binding energy of the photoelectrons depends on the orbital from which they were released, and thus being an element specific, XPS can be used to perform chemical analysis of surfaces. The depth information is higher than with UPS (lower excitation energy) or AES (stronger scattering losses within the solid) and is at about 10 nm.

XPS-Spektrum

A qualitative and quantitative chemical analysis of a surface is possible by XPS, in addition statements about chemical bonds on the surface can be made. From a shift of the binding energy of the electrons of a given orbital (e.g. O 1s) the chemical environment of the electrons of this orbital can be detected.
For example, oxygen atoms may be bonded to carbon or hydrogen atoms, which shifts the binding energy specifically depending on the nature of the resulting molecule (e.g. CO2, CO, OH, H2O). This is also known as "chemical shift". One example is the determination of the oxidation states of iron. This manifests itself in a chemical shift of the Fe 2p orbital. There, e.g. the signal of the Fe 2p3/2 orbital for a pure Fe2O3 is shifted by about 4 eV to a higher binding energy. For a pure FeO, the signal is only shifted by about 3 eV. This means that the III. oxidation state and the II. oxidation state differ by 1 eV.

or more information see here.

 

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