Photoelectron spectroscopy spectrum analysis

In order to examine the chemical state on the carbon surface, x-ray photoelectron spectroscopy (XPS) analysis was performed for both the fluorinated carbon-coated AAO film and the liberated nanotubes from this fluorinated film. In the resulting XPS Cls spectrum of the fluorinated film (Figure 3.11a), the most intense component is the peak assigned to the covalent C-F bond (about 290eV),... 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monocluomatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also infonnation conceming the chemical enviromnent of the atoms in the near-surface region. Thus, it is probably the most popular and usefiil surface analysis teclmique. There are a number of fonus of photoelectron spectroscopy in conuuon use. 

Other techniques utilize various types of radiation for the investigation of polymer surfaces (Fig. 2). X-ray photoelectron spectroscopy (XPS) has been known in surface analysis for approximately 23 years and is widely applied for the analysis of the chemical composition of polymer surfaces. It is more commonly referred to as electron spectroscopy for chemical analysis (ESCA) [22]. It is a very widespread technique for surface analysis since a wide range of information can be obtained. The surface is exposed to monochromatic X-rays from e.g. a rotating anode generator or a synchrotron source and the energy spectrum of electrons emitted... 

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). 

In the case of photoelectron spectroscopy the experimental data provide electronic term values for the excited states of the ion. By the use of Koop-mans theorem,5 estimates of these quantities can be obtained from orbital energies found in calculations on the closed-shell ground state of the molecule. This procedure can give a useful hint at the analysis of a spectrum but is on rather shaky ground theoretically,6 involving a number of assumptions of doubtful validity and giving spurious answers in several well-documented instances. 

Figure 7.38. XPS spectrum of an ionic liquid, [EMIM][Tf2N], detailing the C(ls) and N(ls) regions. Since there are no peaks from the Au substrate, the film thickness is hkely >10nm. Also shown (right) is the comparison between XPS, ultraviolet photoelectron spectroscopy (UPS, Hel = 21.2eV, Hell = 40.8 eV radiation), and metastable impact electron spectroscopy (MIES). Whereas XPS and UPS provide information from the first few monolayers of a sample, MIES is used for zero-depth (surface only) analysis, since the probe atoms are excited He atoms that interact with only the topmost layer of sample. Full interpretations for these spectra may be found in the original work Hofft, O. Bahr, S. Himmer-lich, M. Krischok, S. Schaefer, J. A. Kempter, V. Langmuir 2006, 22, 7120. Copyright 2006 American Chemical Society.

In the case of catalysts, the preparation of samples for LEISS analysis is similar to the preparation used for photoelectron spectroscopy. As a result of the extreme sensitivity of the surface composition, elimination of contamination plays a more important role in the quality of the spectrum. A gas treatment is often applied in order to prepare a representative surface (for example, of the reduced state). 

X-ray and electron spectroscopies are very useful techniques to study the electronic state and chemical bonding of various kinds of functional materials, such as ceramics and alloys. Since the leading achievement by Siegbahn et al. x-ray photoelectron spectroscopy (XPS) is known to be very efficient for chemical state analysis of matters. They provide information not only on chemical components but also that on the valence electronic state and the chemical bonding of atoms constructing the materials. The direct information on the density of state (DOS) for solid state material can be obtained from XPS of the valence state region. Figure 1 schematically illustrates the relationship between the electronic state of matter and photoelectron spectrum as well as x-ray emission and absorption spectroscopies, and also the characteristics of these spectroscopies. 

DV-Xa molecular orbital calculation is demonstrated to be very efficient for theoretical analysis of the photoelectron and x-ray spectroscopies. For photoelectron spectroscopy, Slater s transition state calculation is very effective to give an accurate peak energy, taking account of the orbital relaxation effect. The more careful analysis including the spin-polarized and the relativistic effects substantially improves the theoretical results for the core level spectrum. By consideration of the photoionization cross section, better theoretical spectrum can be obtained for the valence band structure than the ordinary DOS spectrum. The realistic model cluster reproduce very well the valence state spectrum in details. 

Figure 7.11 Auger spectra of a contaminated tungsten foil acquired in a fixed retarding ratio mode with 0.6% relative resolution (a) direct spectrum and (b) differential spectrum. Elements P, N, O, W, C are indicated. (Reproduced with permission from D. Briggs and J.T. Grant, Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, IM Publications and Surface Spectra Ltd, Chichester. 2003 IM Publications.)...

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