Ultraviolet photoelectron spectroscopy spectra

In ultraviolet photoelectron spectroscopy (UPS or U-PES), the irradiation (usually a He(I) (21.2 eV) or He(II) (40.8 eV) source) causes the displacement of a valence electron. Although an important method of studying the electronic nature of molecules in the gas phase, it is less useful for studying the surfaces of metals, since the valence electrons are in a continuous (conducting) energy band with a spread of about 10 eV. Adsorbed layers can, however, usefully be investigated in terms of the difference between the spectrum following adsorption and that for the clean metal surface. 

The development of gas phase ultraviolet photoelectron spectroscopy (UPS) has proceeded rapidly in the last twelve years. Turner first showed that the process of photoionisation could be used to generate a photoelectron spectrum giving a direct measurement of atomic or molecular ionisation potentials (7). The early results of the Oxford group led to the acceptance of two simple guide lines in the interpretation of such spectra. Firstly the electron kinetic energies were considered to satisfy a modified Einstein relation ... 

Figure 7.38. XPS spectrum of an ionic liquid, [EMIM][Tf2N], detailing the C(ls) and N(ls) regions. Since there are no peaks from the Au substrate, the film thickness is hkely >10nm. Also shown (right) is the comparison between XPS, ultraviolet photoelectron spectroscopy (UPS, Hel = 21.2eV, Hell = 40.8 eV radiation), and metastable impact electron spectroscopy (MIES). Whereas XPS and UPS provide information from the first few monolayers of a sample, MIES is used for zero-depth (surface only) analysis, since the probe atoms are excited He atoms that interact with only the topmost layer of sample. Full interpretations for these spectra may be found in the original work Hofft, O. Bahr, S. Himmer-lich, M. Krischok, S. Schaefer, J. A. Kempter, V. Langmuir 2006, 22, 7120. Copyright 2006 American Chemical Society.

Figure 24.30 The He I photoelectron spectrum of water. (From J. W. Rabelais, Principles of Ultraviolet Photoelectron Spectroscopy. New York Wiley, 1977).

The photoelectron spectra can be generated using a primary beam of ultraviolet radiation (known as ultraviolet photoelectron spectroscopy (UPS)), or X-rays to yield an XPS spectrum. It can also be generated by synchrotron radiation (SR), in which case the electron spectrometer must be attached to the beam line at a synchrotron facility. UPS is essentially a molecular spectroscopy that provides high-resolution valence band spectra, while XPS is used predominantly to study core level transitions, and also to study valence band electrons. 

Photoelectron spectroscopy is a technique whereby electrons directly ejected from the surface region of a solid by incident photons are energy analyzed and the spectrum is then related to the electron energy levels of the system. The field is usually arbitrarily divided into two classes ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The names derive from the energies of the photons used in the... 

Ultraviolet (UV) radiation is generally considered to be nonionizing but at the high-energy end of the UV spectrum, photons will have enough energy to ionize valence electrons in atoms and molecules. Ultraviolet photoelectron spectroscopy (UPS) is an example where a helium discharge lamp emits radiation at 21.2 eV (58.5 nm, 5.12 x 10 Hz) and this radiation examines the valence electrons in molecular orbitals. 

Although valence band information could be acquired by conventional X-ray sources, analysis of the valence band region is not as simple as the core region, since all the components in the sample contribute in this narrow region (with E of 30 eV or less). Due to the broad line width of conventional X-ray sources and the low ionization cross section. X-ray-excited valence band spectroscopy is less commonly used for surface analysis. Instead, ultraviolet sources (e.g.. He I and He II) are adopted to acquire the valence band spectra, a surface technique called ultraviolet photoelectron spectroscopy (UPS). He I and He n resonance lines have inherently narrow widths of only a few meVs and high ionization cross sections in the valence band. This technique is widely used in the study of adsorption phenomena and valence band structure of metals, alloys, and semiconductors. Work functions can be derived from the Fermi level and the secondary electron (SE) cutoff of the UPS spectrum. 

While many spectroscopic studies have been published on dimers, the most extensive polymer studies have been with Ag, Na, and Cu clusters. As might be expected much of the interest in silver relates to the photographic process where it appears that a four-atom silver cluster on a silver halide surface leads to reduction by developer, whereas a three-atom cluster does not. The electron spin resonance (ESR) spectrum of sodium in argon confirms that the trimer is covalently bonded and not an equilateral triangle. Ultraviolet photoelectron spectroscopy (UPS) of Cu clusters indicates that the d band is separate from the s band, unlike in the bulk or in the Xa calculations mentioned earlier. 

The electronic and chemical structure of poly(67) was studied experimentally by X-ray and ultraviolet photoelectron spectroscopy, and theoretically by quantum-chemical calculations [122]. X-ray spectroscopy showed that the thin films were chemically pure with no oxygen present. The remarkably well-resolved S2p spectrum indicated the quality of the polymer material to be excellent (minimum of chemical and spatial... 

In vacuum ultraviolet photoelectron spectroscopy (UPS), the sample atom or molecule is exposed to radiation in the vacuum ultraviolet region of the electromagnetic spectrum. A readily available source of radiation is the helium discharge lamp, whieh produces a sharp Hel line at 21.2 eV. Since the energy required for photoionization of sets of valence electrons is in the vicinity of 6 eV to this energy, we obtain a polyenergetic emission of electrons described by the Einstein relation... 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monocluomatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also infonnation conceming the chemical enviromnent of the atoms in the near-surface region. Thus, it is probably the most popular and usefiil surface analysis teclmique. There are a number of fonus of photoelectron spectroscopy in conuuon use. 

Figure 8.6 The He I ultraviolet photoelectron spectrum of argon. (Reproduced from Turner, D. W., Baker, C., Baker, A. D. and Brundle, C. R., Molecular Photoelectron Spectroscopy, p. 41, John Wiley, London, 1970)...

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