Shake-off electrons

In addition to resonant conversion. Auger cascades, and shake-off electrons, there are other secondary mechanisms that can lead to the generation of resonant electrons. Thus, electrons coming from the photoionization of an atom hit by resonant gamma or X-rays originated during the nuclear or atomic relaxation of the Mossbauer isotope (and usually denoted as GPE and XPE, respectively) are also electrons carrying resonant information and, consequently, contribute to the Mossbauer spectrum [4]. 

Experimental data show a sharp peak in the number of electrons (related to Mossbauer events) at energies below 20 eV. These electrons supply information on a surface layer to a depth of --5 nm. The detection of very low energy electrons offers the advantage of short data acquisition times (-77% of the electrons emitted from the Fe atom are low-energy Auger and shake-off electrons), and increases surface sensitivity compared to established procedures relying on the collection of electrons near 7.3 keV. CEMS detectors and techniques are summarized in Table 2. 

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... 

In some cases, a valence electron can be completely ionized, resulting in vacancies in both the core and valence levels. In those cases, weak peaks referred to as shake-off satellites are also observed at binding energies a few electron volts higher than the photoelectron peak. Such cases are, however, not very common. 

The Si(k) term takes into account amplitude reduction due to many-body effects and includes losses in the photoelectron energy due to electron shake-up (excitation of other electrons in the absorber) or shake-off (ionization of low-binding-energy electrons in the absorber) processes. 

At 2000 K there is sufficient energy to make the H2 molecules dissociate, breaking the chemical bond the core density is of order 1026 m-3 and the total diameter of the star is of order 200 AU or about the size of the entire solar system. The temperature rise increases the molecular dissociation, promoting electrons within the hydrogen atoms until ionisation occurs. Finally, at 106 K the bare protons are colliding with sufficient energy to induce nuclear fusion processes and the protostar develops a solar wind. The solar wind constitutes outbursts of material that shake off the dust jacket and the star begins to shine. 

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. 

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. 

Core electron ejection normally yields only one primary final state (aside from shake-up and shake-off states). However, if there are unpaired valence electrons, more than one final state can be formed because exchange interaction affects the spin-up and spin-down electrons differently. If a core s electron is ejected, two final states are formed. If a core electron of higher angular momentum, such as a 2p electron, is ejected, a large number of multiplet states can result. In this case it is difficult to resolve the separate states, and the usual effect of unpaired valence electrons is... 

First, satellite structure on the high binding energy side of, for example, an XPS core-level line (or peak ) corresponds to so-called shake-up (referred to below as s.u. ) and shake-ofF2S-29 effects, the former of which is illustrated, by M+, in Fig. 3.1. Shake-off is just shake-up to the continuum rather than to an unoccupied molecular state. Considerations of (1) are important in comparisons with the results of model calculations while (2) is of use as an indication of the electronic transitions in the molecules under study, an example of which is found in studies of the early stages of interface formation, i.e., the interactions of reactive metal atoms with conjugated polymer surfaces. Since use will be made of these effects in subsequent chapters, they are outlined briefly below. 

Shake-up and shake off losses are final state effects, which arise when the photoelectron imparts energy to another electron of the atom. Ultimately, this electron will be in a higher unoccupied state (shake-up) or in an unbound state... 

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