Photoelectrons, energy analysis

X-ray Photoelectron Spectroscopy analysis of the samples was performed with a Surface Science Instruments spectrometer (SSI 100) with a resolution (FWHM Au 4f7/2) of 1.0 eV. The X-ray beam was a monochromatised AlKa radiation (1486.6 eV). A charge neutraliser (flood gun) was adjusted at an energy of 6 eV. As the Cls spectra of these compounds were very complex, the binding energies were referenced to the binding energy of Ols, considered experimentally to be at 531.8 eV [8). 

Hammer, G.E. and Drzal, L.T. (1980). Graphite fiber surface analysis by X-ray photoelectron spectroscopy and polar/dispersive free energy analysis. Application of Surf. Sci.. 4, 340-355. 

Full derivations of the theory were presented by Lee and Pendry and Ashley and Doniach in 1975. They showed that a complete quantitative description of th EXAFS process was possible and that accurate bond lengths and coordination numbers could be extracted from the analysis of EXAFS data. Lee and Pendry also showed that at high photoelectron energies, the curvature of the electron wave can be neglected and thus the theory can be greatly simplified into what has beeome known as the plane-wave approximation. This approximation results in an expression equivalent to that derived by Stem semi-empirically ... 

Each photoelectron has an energy determined by the difference between the energy of the incident X-ray photon and that of the electronic level initially occupied by the ejected electron. The photoelectron energy spectrum is the basis of a method called ESCA (Electron spectroscopy for the chemical analysis). 

The electron affinity can also be deduced from the measurement of the spectrum of the photoelectron emission with monochromatic UV light. This technique is ultra-violet (UV) photoelectron emission spectroscopy (or UV photoemission spectroscopy or UPS). The UPS technique involves directing monochromatic UV light to the sample to excite electrons from the valence band into the conduction band of the semiconductor. Since the process occurs near the surface, electrons excited above the vacuum level can be emitted into vacuum. The energy analysis of the photoemitted electrons is the photoemission spectrum. The process is often described in terms of a three step model [8], The first step is the photoexcitation of the valence band electrons into the conduction band, the second step is the transmission to the surface and the third step is the electron emission at the surface. The technique of UPS is probably most often employed to examine the electronic states near the valence band minimum. 

An analysis chamber fitted with an X-ray source and a photoelectron energy analyser... 

Energy analysis of the photoelectrons is carried out by a spectrometer of which there are two types cylindrical and spherical sector. In both cases, they are in the form of a capacitor that focuses electrons with the same selected energy level to a detection point, spectrum acquisition is carried out sequentially by scanning the selected energy. 

The LEISS technique has been installed in the majority of cases as an accessory to X-ray pho-toclcctron spectroscopy (XPS). In fact, LEISS provides information with a depth resolution of an atomic layer, supplementing photoelectron surface area analysis which probes a thickness of several nanometres. As with XPS, this technique requires maintaining the sample under vacuum. Moreover, the detection system can be the same because energy analysis of ions only entails a reversal of the polarity on the electron spectrometer plates. Thus the additional equipment comprises an ion gun supplying a monochromatic beam at energy levels of around one kilovolt (Fig. 6.3). 

Figure 5.15 The experimental arrangement of a photoelectron photoion coincidence (PEPICO) set-up. Electron energy analysis is accomplished by an electrostatic energy analyzer, by electron TOP when the light source is pulsed, or by angular discrimination against energetic electrons. Taken with permission from Baer (1986).

Equation (3.1) indicates that XPS is capable of elemental analysis since no two atoms in the Periodic Table exhibit the same set of BEs . An XPS spectrum is a plot of the photoelectron intensity versus the kinetic (or binding) energy of the photoelectrons. Qualitative analysis can easily be performed by wide scans, in which all the available KE range is explored. Higher spectral resolution and more quantitative analysis are performed by detailed or narrow KE (or BE) scans. 

The third type of spectrometer, which uses retarding grids and electronic differentiation for energy analysis, is rather uncommon in conventional photoelectron spectroscopy. The reader is referred for further details to texts on LEED and Auger spectroscopy. 

---