Auger emission

Thus in all X-ray photoelectron spectra, features appear as a result of both photoemission and Auger emission. In XPS, the Auger features can be useful but are not central to the technique, whereas in AES (see Sect. 2.2), Eq. (2.2) forms the basis of the technique. 

FIGURE 27.41 Electronic diagram of the Auger emission process. 

The ionized atom that remains after the removal of the core hole electron is in a highly excited state and will rapidly relax back to a lower energy state by one of two routes, namely X-ray fluorescence (Section 5.1.2) or by transferring the energy to an electron in another orbit, which, if it has sufficient energy, will be ejected into the vacuum as Auger emission. An example of the latter process is illustrated in Figure 5.29. 

The total Auger emission rises rapidly as the angle of incidence of the electron beam a is increased, and this leads to a change in intensity with topography in imaging. To make the image intensity approximately quantitative, Prutton et al. (1983) have proposed the function ... 

Ion beam spectrochemical analysis Auger emission spectroscopy Scanning electron microscopy (SEM) Electron microprobe (EMPA) Particle-induced X-ray emission spectroscopy (PIXE)... 

The transitions involved in Auger emission are illustrated in Fig. 10. The primary process is the ionization of an inner shell by bombardment with electrons. The vacancy is then filled by an electron from an outer shell, and the energy released can either appear as an X-ray quantum, or... 

Carbon deposition from CO on a cobalt catalyst at low pressures is known to be a structure-sensitive process. CO is adsorbed molecularly on the low index surfaces (Co (0001)), but its dissociation occurs on the Co (1012), Co (1120), and polycrystalline surfaces.5762 Deposition of carbon on Co (1012) and the probable formation of Co3C have been established by Auger emission spectroscopy (AES) and low-energy electron diffraction (LEED) techniques.66... 

Auger emission spectroscopy Surface Ciliberto and Spoto (2003)... 

Resonance photoemission measurements have been recently made for U metal , and show indeed a resonant enhancement of the satelUte at 2.3 eV only for the threshold energy (5 A i2. hv = 94 eV) (Fig. 15). In addition the main peak at Ep shows the expected off-resonance behaviour. Further support for such an interpretation of the satellite is given by the analysis of the photon excited Auger emission. This is shown to be composed of two different bands also separated by 2.3 eV and due to the two screening channels by 5 f or 6 d states ... 

Auger emission to neutralize incoming ions leaves the solid surface in an excited state relaxation of the surface results in secondary electron generation (23, 24). Secondary electrons are ejected when high-energy ions, electrons, or neutral species strike the solid surface. These electrons enhance the electron density in the plasma and can alter the plasma chemistry near a solid surface. Radiation impingement on a surface can induce a number of phenomena that depend upon the bombardment flux and energy. 

Figure 2. Scheme of the Auger process. A valence-level involved Auger emission is illustrated here, but the two electrons involved also could have come from core level, 4, provided ts — 2c( > 0. 

Analytical surface techniques such as Auger Emission Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS) analysis are extremely useful in identifying the chemistry of the solid surfaces (Buckley, 1981 Briggs and Seah, 1990). Table 5.7 is a summary of the XPS spectra data for rollering surfaces in oil containing dibenzyl disulfide under various conditions ... 

A fine - structure technique (see EXAFS). Core holes are excited by monoenergetic electrons. The modulation in the excitation cross section as the beam energy is varied may be monitored through absorption, fluorescence, or Auger emission. 

Slow ionized atoms, usually He+, strike a surface, where they are neutralized in a two - electron process that can eject a surface -electron -a process similar to Auger emission from the valence band. The ejected electrons are detected as a function of energy, and the surface density of states can be determined from the energy distribution. The interpretation is more complicated than for SPI or UPS. 

Fig. 8a-c. Photoemission of a primary electron with subsequent Auger emission... 

Exotic atoms are produced by stopping a beam of negatively charged particles like muons, pions, or antiprotons in a target, where they are captured in the Coulomb potential of the atoms at high principal quantum numbers n. These systems deexcite mainly by fast Auger emission of electrons in the upper part of the atomic cascade and more and more by X-radiation for lower-lying states. 

Near-edge X-ray absorption spectroscopy and techniques related to it by an extension of the photoelectron energy range or a modification in the experimental detection mode (EXAFS, surface XAS via Auger emission and... 

In several techniques an additional process associated with one of the four steps generates the detected particles, e.g. Auger emission in Auger-SEXAFS or ion emission in PSD-SEXAFS. This additional process does not normally provide structure sensitivity, but produces particles that are more conveniently detected. We shall include such processes in the discussion of step 4. 

Some techniques also involve a well-defined incident electron beam, even though the primary process at some point imparts an arbitrary parallel momentum to the electrons. This happens, for example, with energy loss in HREELS and ILEED, with diffuse scattering in LEED and with Auger emission in ARAES. In these cases the direction of the electrons leaving the surface has an arbitrary relationship to the incident beam direction. Up to the primary process, however, conventional LEED can be applied in the plane-wave representation, at least in the ordered part of the surface, using the finite set of plane waves defined by the direction of incidence. 

The calculation of PAES intensities largely reduces to the calculation core annihilation probabilities for positrons in the surface state [11]. This follows from the fact that almost all of the core hole excitations of the outer cores relax via Auger emission and that almost all of the positrons incident at low energies become trapped in a surface state before annihilation. First-principles calculations of the positron states and positron annihilation characteristics at metal and semiconductor surfaces are based on a treatment of a positron as a single charged particle trapped in a "correlation well" in the proximity of surface atoms. The calculations were performed within a modified superimposed-atom method using the corrugated-mirror model of Nieminen and Puska [12]. 

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