Photoelectron analysers

Photoelectron Analysers. Electrostatic, TOF, magnetic bottle and threshold electron analysers have all been used. The electrostatic analysers, which are similar to those in the classical PES set-up, have the advantage of an optimized design and ready commercial availability. The electron energy is a function of the analyser voltage, and the resolution... 

Because a set of binding energies is characteristic for an element, XPS can analyse chemical composition. Almost all photoelectrons used in laboratory XPS have kinetic energies in the range of 0.2 to 1.5 keV, and probe the outer layers of tire sample. The mean free path of electrons in elemental solids depends on the kinetic energy. Optimum surface sensitivity is achieved with electrons at kinetic energies of 50-250 eV, where about 50% of the electrons come from the outennost layer. 

Owing to the limited escape depth of photoelectrons, the surface sensitivity of XPS can be enlianced by placing the analyser at an angle to the surface nonnal (the so-called take-off angle of the photoelectrons). This can be used to detemiine the thickness of homogeneous overlayers on a substrate. 

Measurement of the kinetic energy of the photoelectrons (Equation 8.3) involves measurement of their velocity. This has been achieved by various types of analysers and Figure 8.5 shows four of them. 

Mirovsky Y, Tenne R, Cahen D, Sawatzky G, Polak M (1985) Ternary chalcogenide-based photoelectrochemical cells V. Surface Analyses of the CulnX2/aqueous polysulfide interface (X = S, Se) by X-ray photoelectron spectroscopy Absence of Se/S exchange in the CulnSe2/Sn system. J Electrochem Soc 132 1070-1076... 

The XPS technique is highly surface specific due to the short range of the photoelectrons that are excited from the solid detection of electrons gives information from a region between 1.5 and 4.0 nm in from the surface. The minimum sample mass that can be analysed is 10 8kg, while 10-10 to 10-12kg of an element can be detected. Usually samples of 10-100 mg are taken. 

The energy of the photoelectrons leaving the sample are determined using a Concentric Hemispherical Analyser (CHA), and this gives a spectrum with a series of peaks whose energy values are characteristic of each element. A schematic diagram of a CHA is shown in Figure 2.3. 

The energies of the Auger electrons leaving the sample are determined in a manner similar to that employed for photoelectrons already described in chapter 2 Section 4. Modern instruments nearly always incorporate cylindrical mirror analysers (CMA) because their high transmission efficiency leads to better signal-to-noise ratios than the CHA already described. 

The authors thank A. Appelbaum for Auger analyses, J. M. Gibson for transmission electron microscopy, S. B. Dicenzo for X-ray photoelectron spectroscopy and G. K. Celler and L. E. Trimble for use of their laser. 

Three-membered rings. Gas-phase pyrolysis of 20 has been carried out and analysed by photoelectron spectroscopy.216 Selenoketone eliminates molecular nitrogen to form alkylideneselenirane, which decomposes at higher temperature (Scheme 36). 

Infra-red, microwave, and X-ray photoelectron spectroscopy Infra-red and ultra-violet spectroscopy has been widely used for investigating the structure of intermolecularly hydrogen-bonded complexes in the solid state (Novak, 1974) and in solution (Zundel, 1976, 1978 Clements et al., 1971a,b,c Pawlak et al., 1984). By analysing the infra-red spectra of equimolar liquid mixtures of amines with formic or acetic acid, the relative importance of structures [10] and [11] was estimated (Lindemann and Zundel, 1977). It was proposed that [10] and [11] make equal contributions to the observed structure of the complex when the p -value of the carboxylic acid is approximately two units lower than that of the protonated amine. 

Table 6.4 shows the composition corresponding to different PtSn catalysts submitted to XPS and EXAFS/XANES analyses, while Table 6.5 gives XPS results giving the position of all of the main photoelectron peaks after referencing them to the C Is BE of 284.6eV [30, 62]. Figure 6.8 depicts XPS spectra of the Sn3d5/2 level for the tin-modified catalysts. 

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