Final state effects

In reality, while the photoelectron is leaving the atom, the other electrons respond to the hole being created. The responses, known as final state effects., often lead to additional features in the XPS spectrum, some of which are useftil analytically. 

At the same time, the observed BE shifts can also be interpreted solely in terms of final state effects [95]. If one... 

Although the dynamic final state effect can also explain the broadening of the core levels, the expected shift for the Fermi-edge and the core levels should still be quite similar. In fact, in numerous experiments [83,84,86,96,100,101] significantly different shifts were observed for the different spectral elements, which indicates that beyond the final state effects - which probably still give important contributions to shifts - initial state effects also have to be considered. 

On the other hand, the significantly different shift of the Ag core levels and the Fermi edge suggests that the observed changes cannot be explained by a simple final state effect. In addition, taking into account that the BE shift of the Ag 3d levels can be separated into a AF initial state contribution and a h.R final state contribution [96,164]... 

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]. 

As in the parent binary phosphides MV, the metal 2p3 p XPS spectra for the mixed-metal phosphides M XM XV exhibit asymmetric lineshapes originating from final state effects [34] involving the metal-metal bonding network. Virtually no changes in BE are observed relative to the binary phosphides MV or the elemental metals... 

In order to appreciate the meaning of a binding energy, it is necessary to consider final state effects. In practice we use XPS data as if they were characteristic of the atoms as they are before the photoemission event takes place. We must realize that this is not correct photoemission data represent a state from which an electron has just left. Let us analyze the event in more detail. 

Shake-up and shake-off losses are final state effects, which arise when the... 

Of direct interest for photoemission of supported catalysts is that similar increases in the width of d-bands have been observed by Mason in UPS spectra of small metal particles deposited on amorphous carbon and silica substrates [48]. Theoretical calculations by Baetzold et al. [49] indicate that the bulk density of states is reached if Ag particles contain about 150 atoms, which corresponds to a hemispherical particle 2 nm in diameter. Concomitant with the appearance of narrowed d-bands in small particles is the occurrence of an increase in core level binding energies of up to 1 eV. The effect is mainly an initial and only partly a final state effect [48], although many authors have invoked final state - core hole screening effects as the only reason for the increased binding energy. 

The term S0 k) in (6-9) is a correction for relaxation or final state effects in the emitting atom, such as the shake-up, shake-off and plasmon excitations discussed in Chapter 3. The result of these processes is that some absorbed X-ray quanta of energy hv are converted not into photoelectrons of kinetic energy hv-Eb, but into electrons with lower kinetic energy as well. 

Final state effects particularly relevant in photoemission from actinide solids, are now briefly reviewed. 

Very important final state effects are those which are described in the final state multiplet theory. They occur in cores containing unsaturated, localized shells (open-shell cores). The theory has been successfully apphed, e.g., in the interpretation of the spectral response from the 4f levels of lanthanides . ... 

Photoelectron spectroscopy has long be considered as to be able to provide a photographic picture of the one-electron density of state of solids. In reality, the spectra of actinide solids (as of other narrow band solids) need very often more than this naive interpretation. In the case of 5 f response, final state effects are found to provide useful information even in the case of metals, as illustrated in this chapter. The general conclusion that the photoelectron spectroscopic response depends on many-electron excited final states as much as it depends on the initial states, when narrow bands are involved, must be emphasized. This points to the necessity both of better final state models and of band calculations giving reliable pictures of conduction bands. 

Figure 2.21 UVPS (Hell spectra) in the valence-band region showing final-state effects (a) Sm (b) Tb. The valence band of Sm shows the coexistence of both divalent and trivalent species due to valence instability (mixed valence). (After Rao Sarma, 1982.)...

The surface core level shift is defined as the shift in the core level binding energy for a surface atom relative to that of a bulk atom. Different theoretical approaches have been used to calculate surface shifts5,9 and for metals it has recently been shown21 that both initial- and final-state effects have to be included in ab initio calculations to obtain consistent agreement between experimental and calculated results. The basic assumption in this theoretical approach is that the final state is completely screened so the... 

Another final-state effect which gives rise to an increase in the number of photoemission peaks is multiplet splitting. If the valence levels contain unpaired... 

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