Valence electron energy loss spectroscopy

The trend which arises from a consideration of Eq. (17.2) is that the lower the value of the Hamaker constant, A, the higher the equilibrium film thickness (Clarke, 1987). Striking confirmation experimentally of such a trend has come from work in which the local Hamaker constants in silicon nitride ceramics have been determined from spatially resolved-valence electron energy-loss spectroscopy (French et al 1998). Conversely, if A is too large, then there will be no thickness Z for which Eq. (17.2) is satisfied. 

Howe, J. M. and Oleshko, V. R, Application of valence electron energy-loss spectroscopy and plasmon energy mapping for determining material properties at the nanoscale. Journal of Electron Microscopy, S3 (4), 2004, 339-351. 

The difficulty in calculating Ah from the above equations arises from the necessity that the dielectric response function e"(q)) of all materials involved must be known for the full range of relevant frequencies v = q)/2jt, from the infrared to the far ultraviolet. Lack of this information was a critical obstacle for the practical use of the Lifshitz theory. This issue was solved first by using approaches that constructed a function of e (ico) by interpolating the limited amount of spectral data available. Today, new experimental methods enable a much more detailed determination of e(uo) resulting in more precise values of Ah- These methods are primarily vacuum ultraviolet (VUV) spectroscopy [24] and valence electron energy loss spectroscopy (VEELS) that can be carried out in electron microscopes [25]. 

How then, can one recover some quantity that scales with the local charge on the metal atoms if their valence electrons are inherently delocalized Beyond the asymmetric lineshape of the metal 2p3/2 peak, there is also a distinct satellite structure seen in the spectra for CoP and elemental Co. From reflection electron energy loss spectroscopy (REELS), we have determined that this satellite structure originates from plasmon loss events (instead of a two-core-hole final state effect as previously thought [67,68]) in which exiting photoelectrons lose some of their energy to valence electrons of atoms near the surface of the solid [58]. The intensity of these satellite peaks (relative to the main peak) is weaker in CoP than in elemental Co. This implies that the Co atoms have fewer valence electrons in CoP than in elemental Co, that is, they are definitely cationic, notwithstanding the lack of a BE shift. For the other compounds in the MP (M = Cr, Mn, Fe) series, the satellite structure is probably too weak to be observed, but solid solutions Coi -xMxl> and CoAs i yPv do show this feature (vide infra) [60,61]. 

Extensive discussion on the ionization potentials of 1,2,5-thiadiazole and its derivatives can be found in CHEC(1984) and CHEC-II(1996) <1984CHEC(6)513, 1996CHEC-II(4)355>. Hel photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, S2s, Cls and Nls edges, and Sis synchrotron radiation photoabsorption spectroscopy were used to probe the occupied and unoccupied valence levels of benzothiadiazole 2 <1991MI165>. 

The electron-energy-loss spectroscopy (EELS) was performed in transmission with a primary beam energy of 170 keV in a purpose-built UHV spectrometer described in detail elsewhere [5]. For the valence level excitations and elastic scattering (electron diffraction) data the momentum resolution of the instrument was set to 0.04 A 1 with an energy resolution of 90-140 meV. The core level excitations were performed with a momentum and energy resolution of 0.2 A"1 and 90-140 meV, respectively. All EELS experiments were conducted at room temperature. 

Experimental data can be obtained by ultra-violet absorption spectroscopy, electron energy loss spectroscopy and photoelectron spectroscopy. UV absorption and EELs have been described briefly in Chapter 3. The former provides information only about the band-gap, while EELs gives more general information about the conduction bands. Both X-rays and UV photons can be used to generate photoelectrons these two methods are given the acronyms XPS and UPS. The energy spectrum of the emitted electrons provides information about the density of electron states in the valence bands. In principle the size of the band gap can be obtained, but care must be taken as the absolute energy... 

Surface Relaxation, Mixed Valence State and Electron Energy Loss Spectroscopy. 129... 

Valence band electron energy loss spectroscopy (EELS) of oxide superconductors... 

Tafto J, Krivanek OL (1982) Site-specific valence determination by electron energy-loss spectroscopy. Phys Rev Lett 48 560-563... 

Electronic transitions within the valence shell of atoms and molecules appear in the energy-loss spectrum from a few electron volts up to, and somewhat beyond, the first ionization energy. Valence-shell electron spectroscopy employs incident electron energies from the threshold required for excitation up to many kiloelectron volts. The energy resolution is usually sufficient to observe vibrational structure within the Franck-Condon envelope of an electronic transition. The sample in valence-shell electron energy-loss spectroscopy is most often in the gas phase at a sufficiently low pressure to avoid multiple scattering of the... 

Smith , DeWitt JG, Hedman B, Hodgson (1994) Sulfur and chlorine -edge X-ray absorption spectroscopic studies of photographic materials. J Am Chem Soc 116 3836-3847 Sodhi RNS, Brion CE (1985a) High resolution carbon Is and valence shell electronic excitation spectra of trans-1,3-butadiene and allene studied by electron energy loss spectroscopy. J Electron Spec Rel Phen 37 1-21... 

Sze KH, Brion CE (1991) Inner-shell and valence-shell electronic excitation of cyclopropane and ethylene oxide by high resolution electron energy loss spectroscopy. J Electron Spectrosc Relat Phenom 57 117-135... 

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