Photoelectron sample environment

Modem instrumentation has improved substantially in recent years, which has enabled the measurement of XPS spectra of superior resolution necessary to reveal the small BE shifts present in highly covalent compounds such as those studied here. In a laboratory-based photoelectron spectrometer, a radiation source generates photons that bombard the sample, ejecting photoelectrons from the surface that are transported within a vacuum chamber to a detector (Fig. 2). The vacuum chamber is required to minimize the loss of electrons by absorption in air and, if a very high quality vacuum environment is provided (as is the case with modem instruments), the surface contamination is minimized so that the properties of the bulk material are more readily determined. 

X-ray photoelectron spectroscopy also provides information on the chemical composition of a surface. An incoming photon causes electrons to be emitted from atomic core levels, which are then analyzed as a function of kinetic energy. The shifts of these core-level energies provide information about the chemical environment surrounding the excited atom. This information also includes changes in the oxidation state of the sample. 

In XPS, on the other hand, photoelectrons, which are emitted when the sample surface is irradiated with a beam of x-rays, are analyzed. The emitted photoelectrons have discrete binding energies that are dependent on both the identity of the parent element and its chemical environment in the surface. Therefore, both the concentration and the chemical state of an element in the surface can be determined. Two advantages of XPS are that the incident x-ray beam is practically harmless to the surface and it also does not induce charging effects, so that the surface chemistry of adhesives and other insulators can readily be investigated 171 ... 

X-ray Photoelectron Spectroscopy (XPS) and Laser Ion Induced Mass Analysis (LIMA) were used to investigate samples of catalyst A. These techniques can show the extent of potassium and lithium distribution within individual catalyst pellets. Samples that had been subjected to 50,100, and 1000 hours of steam reforming in a molten carbonate environment were analysed. A fresh sample of the catalyst was also examined for purposes of comparison. 

FIGURE 4 Sections of the gas cell, sample holder, and sample and schemes of sample mounting before (A, B) and after (C, D) modernization of the VG ESCALAB HP photoelectron spectrometer for measurements of catalysts in reactive environments (see text) (1) sample (2) holder (3) tungsten wires (4) feedthroughs (5) insulating ceramic (6) spot weld (7) thermocouples. 

The XPS Measurement. In an XPS spectrometer, the studied material is exposed inside a vacuum chamber to a flux of X-rays (energy 1 keV). The kinetic energy of the photoelectrons ejected from the sample is measured by an appropriate analyzer. This energy is directly related to the binding energy of the electrons inside the sample on a wide scan XPS spectrum, the unscattered electrons result in characteristic peaks their energies serve to identify the elements in the material (atomic composition), to characterize the molecular environment of these atoms (chemical analysis, see inset A of Figure 1), and, by the measurement of the photoelectric lines ratios, to reach some quantitative results. Such type of measurement from the core level peaks can usually be... 

In an extensive series of publications. X-ray photoelectron spectroscopy (XPS or ESCA) has been demonstrated to be an extremely powerful tool for the Investigation of structure, bonding and reactivity In polymeric systems ( )- Its small sample requirement, non destructive nature and ability to study solid samples In their working environment with a minimum of preparation, have made It particularly amenable to the study of crossllnked materials whose Insolubility make them difficult to study by any other technique (7). Indeed a perusal of the literature readily affirms that In the field of plasma polymerization ESCA has played an Important role In recent... 

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