XPS

X-ray ray photoelectron spectroscopy (XPS) characterizes the chemical state and elemental abundance of the near-surface by measuring the kinetic energy and intensity of photoelectrons excited by irradiation of a sample (Raeburn et al. 1997b). Excellent reviews of the principles and instrumentation for XPS can be found in Hochella (1988), Turner and Schreifels (2000), and Tonner et al. (1999). 

The work of Raeburn et al. (1997a,b) used a 2 mm x 250 4,m collection area, which is admittedly much larger than true microbeam techniques (but much smaller than bulk methods ). However, their results compare extremely well with those of wet chemistry and Mossbauer. Given the relatively wide availability of XPS instrumentation, this technique has great potential for future studies. 

X-ray photoeleclron spectroscopy is currently the most widely used surface analytical technique, and is therefore described here in more detail than any of the other techniques. At its inception by SiEGBAHN and coworkers [10] it was called ESC A (electron spectroscopy for chemical analysis), but the name ESCA is now considered too general, since many surface electron spectroscopies exist, and the name given to each one must be precise. Nevertheless, the name ESCA is still used in many places, particularly in industrial laboratories and their publications. Briefly, the reasons for the popularity of XPS are the exceptional combination of compositional and chemical information that it provides, its ease of operation, and the ready availability of commercial equipment. 

The surface to be analyzed is irradiated with soft X-ray photons. When a photon of energy hv interacts with an electron in a level with binding energy E, the entire photon energy is transferred to the electron, with the result that a photoelectron is ejected with kinetic energy 

Obviously hv must be greater than 3- The ejected electron may come from a core level or from the occupied portion of the valence band, but in XPS most attention is focused on electrons in core levels. Since no two elements share the same set of electronic binding energies, measurement of the photoelectron kinetic energies provides an elemental analysis. In addition. Equation (1) indicates that any changes in 3 are reflected in Ek, , which means that changes in the chemical environment of an atom can be followed by monitoring 

Thus in all X-ray photoelectron spectra, features appear due to both photoemission and Auger emission. In XPS, the Auger features can be useful but are not central to the technique, whereas in AES (see Chap. 27.3), Equation (2) forms the basis of the technique. 

Quantum numbers n 1 j Spec- trosco- pic state X-ray suffix x-ray state 

If a photon has sufficient energy, on bombarding atoms or ions it can cause electron emission. To a first approximation (Koopman s theorem) 

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In choosing to transfer heat, say XP, from the system above the pinch to the system below the pinch, as shown in Fig. 6.8a, then above the pinch there is a heat deficit of XP. The only way this can... 

Analogous effects are caused by the inappropriate use of utilities. Utilities are appropriate if they are necessary to satisfy the enthalpy imbalance in that part of the process. Above the pinch in Fig. 6.7a, steam is needed to satisfy the enthalpy imbalance. Figure 6.86 illustrates what happens if inappropriate use of utilities is made and some cooling water is used to cool hot streams above the pinch, say, XP. To satisfy the enthalpy imbalance above the pinch, an import of (Q mjj,+XP) is needed from steam. Overall, (Qcmin+AP) of cooling water is used. ... 

A line of constant Xp is compared with a line of constant Fp in Fig. 7.9. It can be seen that the line of constant Xp avoids the regions of steep slope. 

Figure 7.9 The Xp parameter avoids steep slopes on the Fp curves, whereas minimum Fp does not. (Reprinted from Ahmad, Linnhoff, and Smith, Cost Optimum Heat Exchanger Networks II. Targets and Design for Detailed Capital Cost Models, Computers Chem, Engg., 7 751, 1990 with permission from Elsevier Science, Ltd.)...

Xp is chosen to satisfy the minimum allowable Ft (e.g., for Ft > 0.75, Xp = 0.9 is used). Once the real (noninteger) number of shells is calculated from Eq. (7.14), this is rounded up to the next largest number to obtain the number of shells. 

In practice, the integer number of shells is evaluated from Eq. (7.18) for each side of the pinch. This maintains consistency between achieving maximum energy recovery and the corresponding minimum number of units target Nu- ixs- In summary, the number of shells target can be calculated from the basic stream data and an assumed value of Xp (or equivalently,... 

The Fp correction factor for each enthalpy interval depends both on the assumed value of Xp and the temperatures of the interval on the composite curves. It is possible to modify the simple area target formula to obtain the resulting increased overall area A etwork for a network of 1-2 exchangers ... 

The value of Pi 2 required in each 1-2 shell to satisfy a chosen value of Xp is defined by... 

These expressions define Pn-2n for number of 1-2 shells in series in terms of R and Xp in each shell. The expressions can be used to define the number of 1-2 shells in series required to satisfy a specified value of Xp in each shell for a given R and Pjv 2n- Hence the relationship can be inverted to find the value of N which satisfies Xp exactly in each 1-2 shell in the series ... 

Choosing the number of 1-2 shells in series to be the next largest integer above N ensures a practical exchanger design satisfying Xp. 

Schomaker-StevensoD equation The equation a B = a + fl 0 09 (Xa - X ) relating the bond length to the individual radii rp and Tb of the two atoms concerned and the electronegativities Xp and X of the two atoms concerned in the bond. This relation is only empirical and is not very accurate. 

Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

XPS X-ray photoelectron spectroscopy [131-137] Monoenergetic x-rays eject electrons from various atomic levels the electron energy spectrum is measured Surface composition, oxidation state... 

ESCA Electron spectroscopy for chemical analysis [106, 138-142] Same as XPS Same as XPS... 

PES Photoelectron spectroscopy Same as XPS with UV light Similar to XPS... 

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

Fig. XVin-6. Curve-fitted Mo XPS 3d spectra of a 5 wt% Mo/Ti02 catalyst (a) in the oxidic +6 valence state (b) after reduction at 304°C. Doublets A, B, and C refer to Mo oxidation states +6, +5, and +4, respectively [37]. (Reprinted with permission from American Chemical Society copyright 1974.)...

Fig. XVIII-7. Br(3p) XPS spectra of a 2.2-L dose of DBr on Pt(l 11) before (a) and (b) after UV irradiation. The 182.8-eV peak is due to DBr and the 181.5 peak, to atomic Br. On irradiation the latter peak increases relative to the former [38]. (Reprinted with permission from American Chemical Society, copyright 1992.)...

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]. 

XPS is also often perfonned employing syncln-otron radiation as the excitation source [59]. This technique is sometimes called soft x-ray photoelectron spectroscopy (SXPS) to distinguish it from laboratory XPS. The use of syncluotron radiation has two major advantages (1) a much higher spectral resolution can be achieved and (2) the photon energy of the excitation can be adjusted which, in turn, allows for a particular electron kinetic energy to be selected. 

One of the more recent advances in XPS is the development of photoelectron microscopy [ ]. By either focusing the incident x-ray beam, or by using electrostatic lenses to image a small spot on the sample, spatially-resolved XPS has become feasible. The limits to the spatial resolution are currently of the order of 1 pm, but are expected to improve. This teclmique has many teclmological applications. For example, the chemical makeup of micromechanical and microelectronic devices can be monitored on the scale of the device dimensions. 

Flere P is an integer. By joining the ends Xq and Xp and labelling this v, one sees that... 

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