Plasmon-loss

Figure 4 Experimental low-loss profiles for Mg (10.0)< Ti (17.2), Zr(16.6), and their hydrides MgH2 (14.2), TiH 97 (20.0), and ZrHj g (18.1). The values in parentheses represent the experimental plasmon-loss peak energies in eV.

Dravid et al. examined anisotropy in the electronic structures of CNTs from the viewpoint of momentum-transfer resolved EELS, in addition to the conventional TEM observation of CNTs, cross-seetional TEM and precise analysis by TED [5]. Comparison of the EEL spectra of CNTs with those of graphite shows lower jc peak than that of graphite in the low-loss region (plasmon loss), as shown in Fig. 7(a). It indicates a loss of valence electrons and a change in band gap due to the curved nature of the graphitic sheets. 

Although CNTs showed similar EELS pattern in plasmon-loss and core-loss regions to graphite, SWCNT and fine MWCNT with a diameter less than 5 nm had different features. Furthermore, it has been found out that the angular-dependent EELS along the direction normal to the longitudinal axis of CNT shows stronger contribution from Jt electrons than [Pg.38]

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]. 

Fig. 18 Normalized intensity of plasmon loss peak in the Co 2p3/2 spectrum vs. difference in electronegativity for Co-containing phosphides. The dashed line indicates the value for Co metal. Reprinted with permission from [60], Copyright Elsevier...

Fig. 24 a Co 2p3/2 XPS spectra for some members of the CoAsi- Py series, b Plot of normalized plasmon loss intensity versus y. Reprinted with permission from [61]. Copyright Elsevier... 

The transition-metal and rare-earth core-line XPS spectra show little, if any, BE shifts at all. Nevertheless, information about atomic charge and valence states can be extracted by examining other features in the spectra. The plasmon loss satellite intensity found in the spectra of Co-containing compounds provides a particularly useful handle on the Co charge. The lineshapes of RE spectra are characteristic of their valence state, as seen in the distinction between trivalent and tetravalent cerium in CeFe4Pni2 compounds. 

We have tacitly assumed that the photoemission event occurs sufficiently slowly to ensure that the escaping electron feels the relaxation of the core-ionized atom. This is what we call the adiabatic limit. All relaxation effects on the energetic ground state of the core-ionized atom are accounted for in the kinetic energy of the photoelectron (but not the decay via Auger or fluorescence processes to a ground state ion, which occurs on a slower time scale). At the other extreme, the sudden limit , the photoelectron is emitted immediately after the absorption of the photon before the core-ionized atom relaxes. This is often accompanied by shake-up, shake-off and plasmon loss processes, which give additional peaks in the spectrum. 

Figure 3.23 Energy spectrum of electrons coming from a surface irradiated with a beam of primary electrons. Electrons have lost energy to vibrations and electronic transitions (loss electrons), to collective excitations of the electron sea (plasmon losses) and to all kinds of inelastic processes (secondary electrons). The element-specific Auger electrons appear as small peaks on an intense background and are better visible in a derivative spectrum.

Fig. 3.1-13. Hel photoelectron spectra of Cs and its suboxides (intensities in arbitrary units, dashed curves magnified by a factor of 10), surface plasmon loss (hwSp) and work function (3>) indicated.

This data reduction algorithm has a big advantage Many inelastic scattered electrons contribute to the background which is not peaked at the locations of the Bragg-peaks. Their contribution is automatically subtracted. On the other hand form the plasmon-loss electrons a diffraction pattern that gives no significant different result in the refinement process that follows. A... 

Measurements of the dielectric response from the optical properties and plasmon losses have similar problems. 

After core hole formation, relaxation in the valence orbitals can give rise to promotion of valence electrons into unoccupied levels. If this reorganization is fast, and the energy required for this transition is not available to the primary photoelectron, shake-up satellites can show up on the low kinetic energy (high p) side of the main peak. Further loss lines can be created if the photoelectron passing the solid excites group oscillation of the conduction electrons (plasmon loss). 

Plasmon scattering. The incident electrons lose energy by exciting collective oscillations (called plasmons) of the valence electrons. The energy loss is of the order of 15 eV, and plasmon loss peaks are prominent in the low-loss region of electron energy-loss spectra. 

The plasmon loss peak may be used in several analytical applications. For example carbonaceous materials with high polyaromatic content show a plasmon loss peak of their Cjs photoelectrons at 291.2 eV. This peak is clearly resolved from the Cjs peak [8]. 

The satellite features of Fig, 3 for Ceo resemble those of small molecules at low energy but also mimic the behavior of graphite, glassy carbon, and diamond in the region of the plasmon losses, —30 eV. Here we see a broad feature centered —28 eV below the main line. 

FIG. 3. C Is features for C o referenced to the main line 282.9 eV below the center of the highest-occupied-state feature. Features 2-9 reflect sbakeup structures of the form tc to n or based molecules. Feature 2 probably reflects excitation across the gap of the excited state. Feature 10 reflects a plasmon loss due to excitation of a collective mode of the cluster or the condensed solid. 

Figure 7.10 Examples of several types of satellite peaks in XPS spectra (a) shake-up peaks in a CuO spectrum (b) shake-up peaks and multiplet splitting in a NiO spectrum and (c) plasmon loss peak in a clean A1 spectrum. (Reproduced with permission from J.F. Watts, An Introduction to Surface Analysis by Electron Spectroscopy, Oxford University Press, Oxford. 1990 Royal Microscopy Society.)...

---