Photoelectron plasmon losses

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]. 

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. 

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.

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). 

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]. 

Plasmon loss generates another type of satellite peaks these peaks do not provide useful information but they complicate a spectrum. Plasmon loss refers to the energy loss of a photoelectron because it excites collective vibrations in conduction electrons in a metal. The vibrations require a characteristic amount of energy. Such characteristic amounts of photoelectron energy loss in photoelectrons will generate satellite peaks as shown in Figure 7.10c. Plasmon loss peaks are shown in XPS spectra of clean metal surfaces and also in Auger spectra. 

Associated with prominent features in an Auger spectrum are the same types of energy loss feature, the plasmon losses, that are found associated with photoelectron peaks in an XPS spectrum. As described in Section 27.2.3, plasmon energy losses arise from excitation of modes of collective oscillation of the conduction electrons by outgoing secondary electrons of sufficient energy. Successive plasmon losses suffered by the backscattered... 

In addition to primary features from copper in Eig. 2.7 are small photoelectron peaks at 955 and 1204 eV kinetic energies, arising from the oxygen and carbon Is levels, respectively, because of the presence of some contamination on the surface. Secondary features are X-ray satellite and ghost lines, surface and bulk plasmon energy loss features, shake-up lines, multiplet splitting, shake-off lines, and asymmetries because of asymmetric core levels [2.6]. 

We must also take into account two further factors. First, the fact that the transmission efficiency of the analyzer is a fimction of the kinetic energy (K.E.) of the photoelectrons in the ESCA-3 Vacumn Generators instrument the transmission is inversely proportional to the K.E. of the electrons (3a). Second, photoelectron yields must refer to total yield from a particular ionization process and this need not, for example, be just the area of the relevant peak. Account must be taken of all processes that divert electrons from the primary peak, e.g., shake-up, shake-oflF, and plasmon peaks. In some cases, e.g., emission from the Cu 2P3/2 level, the contribution of additional processes is small but in others, and emission from the Al(2p) shell is an example, the no-loss peak is substantially less than the true Al(2p) emission. 

A variety of processes may take place at this stage, distinguishing the different techniques — photoelectron emission, Auger electron emission, ion emission, energy loss due to core-hole, plasmon or phonon excitation, as well as... 

---