Photoemission, solids

R. G. Egdell, S. Eriksen, W. R. Flavell, Oxygen deficient SnO2(110) and TiO2(110) -a comparative-study by photoemission. Solid State Commun. 1986, 60(10), 835-838. 

Many phenomena in solid-state physics can be understood by resort to energy band calculations. Conductivity trends, photoemission spectra, and optical properties can all be understood by examining the quantum states or energy bands of solids. In addition, electronic structure methods can be used to extract a wide variety of properties such as structural energies, mechanical properties and thennodynamic properties. 

Weaver J H 1992 Eleotronio struotures of Cgg, C g and the fullerides—photoemission and inverse photoemission studies J. Phys. Chem. Solids 53 1433... 

W. E. Spicer. In Survey of Phenomena in Ionized Gases. International Atomic Energy Agency, Vienna, 1968, p. 271. A review of the early photoemission work on solids by the pioneering group in this area. 

An XPS spectrum consists of a plot of N(E)/E, the number of photoelectrons in a fixed small interval of binding energies, versus E. Peaks appear in the spectra at the binding energies of photoelectrons that are ejected from atoms in the solid. Since each photoemission process has a different probability, the peaks characteristic of a particular element can have significantly different intensities. 

Two other types of peaks that can be observed in the XPS spectrum of solid materials are referred to as a shake-up and shake-off satellites. When a core-level electron is ejected from an atom by photoemission, the valence... 

J. W. Davenport. Theory of Photoemission from Molecules in the Gas Phase and on Solid Surfaces. Ph.D. thesis, University of Pennsylvania, 1976. 

Eberhardt, W., Cantor, R., Greuter, F. and Plummer, E.W. (1982) Photoemission from condensed layers of molecular hydrogen on copper and gold. Solid State Communications, 42, 799-802. 

Photoemission of singlet oxygen from the surface of solids... 

In the Introduction the problem of construction of a theoretical model of the metal surface was briefly discussed. If a model that would permit the theoretical description of the chemisorption complex is to be constructed, one must decide which type of the theoretical description of the metal should be used. Two basic approaches exist in the theory of transition metals (48). The first one is based on the assumption that the d-elec-trons are localized either on atoms or in bonds (which is particularly attractive for the discussion of the surface problems). The other is the itinerant approach, based on the collective model of metals (which was particularly successful in explaining the bulk properties of metals). The choice between these two is not easy. Even in contemporary solid state literature the possibility of d-electron localization is still being discussed (49-51). Examples can be found in the literature that discuss the following problems high cohesion energy of transition metals (52), their crystallographic structure (53), magnetic moments of the constituent atoms in alloys (54), optical and photoemission properties (48, 49), and plasma oscillation losses (55). 

In Figure 5, the normalized emission spectra of the two solid hybrid materials, GFP/SBA-15 and GFP/Aerosil , are reported. The shape of the emission profile for GFP/SBA-15 follows closely that of the GFP in buffer solution, whereas the photoemission intensity of GFP/Aerosil is one order of magnitude lower and slightly different in its tale shape (spectra at the actual intensities not reported). This reduction in intensity could be explained by a multilayer arrangement of the protein molecules on the amorphous nanoparticles, which would explain both the difference in emission spectra ("self-quenching effect") and the difference in adsorption amount shown above. 

Figure 4. Photoemission spectra of GFP in buffer solution (dotted line) and GFP/SBA-15 (solid line).

Our discussion of electronic structure has been in terms of band filling only. Of course, there is a lot more to know about band structures. The density of states represents only a highly simplified representation of the actual electronic structure, which ignores the three-dimensional structure of electron states in the crystal lattice. Angle-dependent photoemission gives information on this property of the electrons. The interested reader is referred to standard books on solid state physics [9,10] and photoemission [16,17]. The interpretation of photoemission and X-ray absorption spectra of catalysis-oriented questions, however, is usually done in terms of the electron density of states only. 

Visible light or other electromagnetic radiation incident on a solid, liquid, or gas can liberate electric charges. This is called photoelectricity. Ejection of electrons from the surface is usually called photoemission. Electrons or positive ions formed in a gas as the result of such radiation is called photoionization. Such a process, however, cannot charge a particle directly. The charging process in that case is a direct result of subsequent diffusion. 

CO adsorption, 28 8 metal-alkene surfaces, 29 85-86 metal oxide surfaces, 29 55-92 oxide surface, 28 26 solid surfaces, 29 55-92 surface chemistry, 29 55-92 yield, chemisorbed layer, 29 59-62 factors affecting yield, 29 61 Photoemission... 

One way of experimentally exploring the electronic structure of solids is by means of photoemission spectroscopies such as UPS and X-ray photoelectron spectroscopy (XPS), where photoexcited electrons are analyzed dispersively as a function of their kinetic energy. The electronic structure of the reference material TTF-TCNQ will be extensively discussed in Section 6.1. Figure 1.31 shows the XPS spectra of the S2p core line for (TMTTF)2PF6 (black dots) and BEDT-TTF (grey dots). 

The unoccupied electronic states of a solid can be experimentally explored by different techniques. The most commonly used are inverse photoemission, where low-energy electrons impinge on the surface of the solid, and the photon-based techniques ellipsometry, NEXAFS and constant-initial-state spectroscopy. Results derived from inverse photoemission spectroscopy might be questionable unless low-energy electrons (c. 10-20 eV) and low beam currents are used as in LEED... 

Fig. 4.9. Schematic of photoemission experiments, A beam of incident photons with energy ftto induces electrons to emit from the sample. The photoelectrons are collected by the velocity analyzer and the electron detector at angles 9 and

Using inverse photoemission, the unoccupied electronic states of solid surfaces are being studied. Here, instead of injecting an UV light onto the surface and analyzing the emitted electrons, an electron beam is injected onto the surface and the spectrum of the emitted photons is analyzed. Fig. 4.11 shows a summary of the results of photoemission and inverse photoemission of one of the most exhaustively studied surfaces, W(OOl) [Drube et al. (1986)]. As shown, strong surface states immediately below and above the Fermi level are observed. Both are of a character. 

The problem of first-principles calculations of the electronic structure of solid surface is usually formatted as a problem of slabs, that is, consisting of a few layers of atoms. The translational and two-dimensional point group symmetry further reduce the degrees of freedom. Using modern supercomputers, such first-principles calculations for the electronic structure of solid surfaces have produced remarkably reproducible and accurate results as compared with many experimental measurements, especially angle-resolved photoemission and inverse photoemission. 

Cardona, M., and Ley, L. (1978). Photoemission in Solids I General Principles, Springer-Verlag, Berlin. 

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