Valence spectroscopy

Tery L. Barr, Mengping Yin and Shikha Varma, Detailed x-ray photoelectron spectroscopy valence band and core level studied of select metals oxidations, J. Vac. Set. Technol. A, 10, 2383-2390 (1992). 

The principal use of Auger spectroscopy is in the determination of surface composition, although peak positions are secondarily sensitive to the valence state of the atom. See Refs. 2, 82, and 83 for reviews. 

MDS Metastable deexcitation spectroscopy [119] Same as PI Surface valence-electron states... 

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... 

Ultraviolet photoelectron spectroscopy (UPS) is a variety of photoelectron spectroscopy that is aimed at measuring the valence band, as described in sectionBl.25.2.3. Valence band spectroscopy is best perfonned with photon energies in the range of 20-50 eV. A He discharge lamp, which can produce 21.2 or 40.8 eV photons, is commonly used as the excitation source m the laboratory, or UPS can be perfonned with synchrotron radiation. Note that UPS is sometimes just referred to as photoelectron spectroscopy (PES), or simply valence band photoemission. 

Figure Bl.6.12 Ionization-energy spectrum of carbonyl sulphide obtained by dipole (e, 2e) spectroscopy [18], The incident-electron energy was 3.5 keV, the scattered incident electron was detected in the forward direction and the ejected (ionized) electron detected in coincidence at 54.7° (angular anisotropies cancel at this magic angle ). The energy of the two outgoing electrons was scaimed keeping the net energy loss fixed at 40 eV so that the spectrum is essentially identical to the 40 eV photoabsorption spectrum. Peaks are identified with ionization of valence electrons from the indicated molecular orbitals.

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... 

UPS UV photoelectron spectroscopy Absorption of UV light by an atom, after which a valence electron Is ejected. Chemical bonding, work function... 

Ultraviolet photoelectron spectroscopy (UPS) [2, 3 and 4, 6] differs from XPS in that UV light (He I, 21.2 eV He II, 40.8 eV) is used instead of x-rays. At these low excitmg energies, photoemission is limited to valence electrons. 

Concelcao J, Laaksonen R T, Wang L S, Guo T, Nordlander P and Smalley R E 1995 Photoelectron spectroscopy of transition metal clusters correlation of valence electronic structure to reactivity Rhys. Rev. B 51 4668... 

In absorption spectroscopy a beam of electromagnetic radiation passes through a sample. Much of the radiation is transmitted without a loss in intensity. At selected frequencies, however, the radiation s intensity is attenuated. This process of attenuation is called absorption. Two general requirements must be met if an analyte is to absorb electromagnetic radiation. The first requirement is that there must be a mechanism by which the radiation s electric field or magnetic field interacts with the analyte. For ultraviolet and visible radiation, this interaction involves the electronic energy of valence electrons. A chemical bond s vibrational energy is altered by the absorbance of infrared radiation. A more detailed treatment of this interaction, and its importance in deter-... 

The theory of molecular symmetry provides a satisfying and unifying thread which extends throughout spectroscopy and valence theory. Although it is possible to understand atoms and diatomic molecules without this theory, when it comes to understanding, say, spectroscopic selection rules in polyatomic molecules, molecular symmetry presents a small barrier which must be surmounted. However, for those not needing to progress so far this chapter may be bypassed without too much hindrance. 

The simplest, and perhaps the most important, information derived from photoelectron spectra is the ionization energies for valence and core electrons. Before the development of photoelectron spectroscopy very few of these were known, especially for polyatomic molecules. For core electrons ionization energies were previously unobtainable and illustrate the extent to which core orbitals differ from the pure atomic orbitals pictured in simple valence theory. 

Other techniques in which incident photons excite the surface to produce detected electrons are also Hsted in Table 1. X-ray photoelectron Spectroscopy (xps), which is also known as electron spectroscopy for chemical analysis (esca), is based on the use of x-rays which stimulate atomic core level electron ejection for elemental composition information. Ultraviolet photoelectron spectroscopy (ups) is similar but uses ultraviolet photons instead of x-rays to probe atomic valence level electrons. Photons are used to stimulate desorption of ions in photon stimulated ion angular distribution (psd). Inverse photoemission (ip) occurs when electrons incident on a surface result in photon emission which is then detected. 

Shorter-wavelength radiation promotes transitions between electronic orbitals in atoms and molecules. Valence electrons are excited in the near-uv or visible. At higher energies, in the vacuum uv (vuv), inner-shell transitions begin to occur. Both regions are important to laboratory spectroscopy, but strong absorption by make the vuv unsuitable for atmospheric monitoring. Electronic transitions in molecules are accompanied by stmcture... 

An important property of the surface behaviour of oxides which contain transition metal ions having a number of possible valencies can be revealed by X-ray induced photoelectron spectroscopy. The energy spectrum of tlrese electrons give a direct measure of the binding energies of the valence electrons on the metal ions, from which the charge state can be deduced (Gunarsekaran et al., 1994). 

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