Binding energy, ESCA analysis

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is described in section Bl.25,2.1. The most connnonly employed x-rays are the Mg Ka (1253.6 eV) and the A1 Ka (1486.6 eV) lines, which are produced from a standard x-ray tube. Peaks are seen in XPS spectra that correspond to the bound core-level electrons in the material. The intensity of each peak is proportional to the abundance of the emitting atoms in the near-surface region, while the precise binding energy of each peak depends on the chemical oxidation state and local enviromnent of the emitting atoms. The Perkin-Elmer XPS handbook contains sample spectra of each element and bindmg energies for certain compounds [58]. 

Direct measurement of the absolute binding energy and widths of core (X-ray) and valence (UV) bands. The core levels do not participate in bonding, hence each element gives a characteristic XPS spectrum electron spectroscopy for chemical analysis (ESCA). ESCA gives the elemental composition of the surface of a solid sample (except H), the relative amounts of each element present, its oxidation state and some information on the chemical environment around each element. In addition, it is capable of providing an estimate of the depth of a deposited overlaycr... 

The 5950A ESCA spectrometer is interfaced to a desktop computer for data collection and analysis. Six hundred watt monochromatic A1 Ka X-rays are used to excite the photoelectrons and an electron gun set at 2 eV and 0.3 mAmp is used to reduce sample charging. Peak areas are numerically integrated and then divided by the theoretical photoionization cross-sections (11) to obtain relative atomic compositions. For the supported catalyst samples, all binding energies (BE) are referenced to the A1 2p peak at 75.0 eV, the Si 2p peak at 103.0 eV, or the Ti 2p3/2 peak at 458.5 eV. 

In X-ray photoelectron spectroscopy (XPS or X-PES), the irradiation (usually a Mg K a (1253.6 eV) or A1K a (1486.6 eV) source) causes a core electron to be ejected. This is a more useful technique than UPS for surface studies, since the binding energies of core electrons are characteristic of the elements in question and surface elements can thus be identified by the traditional spectroscopic fingerprinting procedure. In this respect, XPS is sometimes referred to by its alternative name, electron spectroscopy for chemical analysis (ESCA). In order to emphasis the contribution from surface atoms, the X-ray beam is usually set at a grazing angle to the surface. Most of the signal originates from within a nanometre of the surface. 

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