X-ray photoelectrons

Finally, similar effects can be seen in miscible polymer blends where the surface tension correlates with the enrichment of the lower-energy component at the surface as monitored by x-ray photoelectron spectroscopy [104],... 

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... 

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... 

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. 

Powell C J, Jablonski A, Tilinin I S, Tanuma S and Penn D R 1999 Surface sensitivity of Auger-electron spectroscopy and x-ray photoelectron spectroscopy J. Eiectron Spec. Reiat. Phenom. 98-9 1... 

Powell C J 1994 Inelastic interactions of electrons with surfaces applications to Auger-electron spectroscopy and x-ray photoelectron spectroscopy Surf. Sc/. 299-300 34... 

Egelhoff W F Jr 1990 X-ray photoelectron and Auger electron forward scattering a new tool for surface crystallography CRC Crit. Rev. Soiid State Mater. Sc/. 16 213... 

X-ray photoelectron spectroscopy (XPS) is among the most frequently used surface chemical characterization teclmiques. Several excellent books on XPS are available [1, 2, 3, 4, 5, 6 and 7], XPS is based on the photoelectric effect an atom absorbs a photon of energy hv from an x-ray source next, a core or valence electron with bindmg energy is ejected with kinetic energy (figure Bl.25.1) ... 

XPS X-ray photoelectron spectroscopy Absorption of a photon by an atom, followed by the ejection of a core or valence electron with a characteristic binding energy. Composition, oxidation state, dispersion... 

Briggs D and Seah M P (eds) 1983 Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (New York Wiley)... 

Wagner C D, Riggs W M, Davis L E, Moulder J F and Muilenburg G E 1979 Handbook of X-ray Photoelectron Spectroscopy (Eden Prairie, MN Perkin Elmer)... 

Vanderputten D et al 1996 Angle resolved x-ray photoelectron spectroscopic experiments on the full series of molecular [AUgg(PR3) 2 101 clustered. Chem. See. Dalton Trans. 8 1721... 

McFeely and co-workers used soft x-ray photoelectron spectroscopy (SXPS) to measure the changes in binding energies of Si(2p) levels after slight exposure to fluorine atoms via dissociative chemisoriDtion of XeF2 [39]. Using synclirotron radiation at 130 eV as the source enabled extreme surface sensitivity. Since this level is split into a... 

Figure 8.1 Processes occurring in (a) ultraviolet photoelectron spectroscopy (UPS), (b) X-ray photoelectron spectroscopy (XPS) and (c) Auger electron spectroscopy (AES)...

This point is illustrated in Figure 8.13 which shows the X-ray photoelectron spectrum of a 2 1 mixture of CO and CO2 gases obtained with MgXa (1253.7 eV) source radiation. The ionization energy for removal of an electron from the s orbital on a carbon atom, referred to as the C s ionization energy, is 295.8 eV in CO and 297.8 eV in CO2, these being quite comfortably resolved. The O s ionization energy is 541.1 eV in CO and 539.8 eV in CO2, which are also resolved. 

Figure 8.13 The MgATa oxygen Is and carbon Is X-ray photoelectron spectra of a 2 1 mixture of CO and CO2 gases. (Reproduced, with permission, from Allan, C. J. and Siegbahn, K. (November 1971), Publication No. UUIP-754, p. 48, Uppsala University Institute of Physics)...

Question. For X-ray photoelectron spectra of a mixture of acetone and carbon dioxide gases, explain what you would expect to observe regarding the relative ionization energies (binding energies) and intensities in the C Is and O Is spectra. 

Figure 8.14 The monochromatized AlATa carbon Is X-ray photoelectron spectrum of ethyltrifluoroacetate showing the chemical shifts relative to an ionization energy of 291.2 eV (Reproduced, with permission, from Gelius, U., Basilier, E., Svensson, S., Bergmark, T. and Siegbahn, K., J. Electron Spectrosc., 2, 405, 1974)...

The chemical shift is related to the part of the electron density contributed by the valence electrons, ft is a natural extension, therefore, to try to relate changes of chemical shift due to neighbouring atoms to the electronegativities of those atoms. A good illustration of this is provided by the X-ray photoelectron carbon Is spectmm of ethyltrifluoroacetate, CF3COOCH2CH3, in Figure 8.14, obtained with AlXa ionizing radiation which was narrowed with a monochromator. 

Figure 8.15 The carbon Is X-ray photoelectron spectra of furan, pyrrole and thiophene. The sulphur Ip spectrum of thiophene is also shown. (Reproduced with permission from Gelius, U., Allan, C. J., Johansson, G., Siegbahn, H., Allison, D. A. and Siegbahn, K., Physica Scripta, 3, 237, 1971)...

Both UPS and XPS of solids are useful techniques. So far as studies of adsorption by surfaces are concerned we would expect UPS, involving only valence orbitals, to be more sensitive. For example, if we wish to determine whether nitrogen molecules are adsorbed onto an iron surface with the axis of the molecule perpendicular or parallel to the surface it would seem that the valence orbitals would be most affected. This is generally the case but, because ultraviolet photoelectron spectra of solids are considerably broadened, it is the X-ray photoelectron spectra that are usually the most informative. 

Figure 8.18 shows an X-ray photoelectron spectrum of gold foil with mercury absorbed onto the surface. Both the gold and mercury doublets result from the removal of a 4/ electron leaving /2 and /2 core states for which L = 3, S = and J = or Less than 0.1 per cent of a monolayer of mercury on a gold surface can be detected in this way. 

In Figure 8.19 is shown the X-ray photoelectron spectrum of Cu, Pd and a 60 per cent Cu and 40 per cent Pd alloy (having a face-centred cubic lattice). In the Cu spectrum one of the peaks due to the removal of a 2p core electron, the one resulting from the creation of a /2 core state, is shown (the one resulting from the 1/2 state is outside the range of the figure). 

Figure 8.20 Nitrogen Is and oxygen Is X-ray photoelectron spectra of nitric oxide (NO) adsorbed on an iron surface. 1, Fe surface at 85 K 2, exposed at 85 K to NO at 2.65 x 10 Pa for 80 s 3, as for 2 but exposed for 200 s 4, as for 2 but exposed for 480 s 5, after warming to 280 K. (Reproduced, with permission, from Kishi, K. and Roberts, M. W., Proc. R. Soc. Land., A352, 289, 1976)...

Figure 8.25 shows the AXn,m. ii,iii Auger spectrum of a gaseous mixture of SFe, SO2 and OCS, all clearly resolved. The three intense peaks are due to sulphur in a >2 core state, but there are three weak peaks due to a core state also. The S 2p X-ray photoelectron spectrum of a mixture of the same gases is shown for comparison, each of the three doublets being due to sulphur in a 1/2 or 3/2 core state. 

Barr, T. L. (1994) Modern ESCA The Principles and Practice of X-ray Photoelectron Spectroscopy, CRC Press, Boca Raton, FL. 

Briggs, D. (Ed.) (1994) Practical Surface Analysis Auger and X-ray Photoelectron Spectroscopy, John Wiley, Chichester. 

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