Excitation of Inner-shell Electrons
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Inner shell excitations occur at energies:
         ΔE ≥ EF - EB ----------------------------------- (3960)
where,
         ΔE -- The energy loss
         EF -- The Fermi level energy
         EB -- Binding energy of the inner shell
The EELS feature corresponds to "core loss edges" or "edge onset" (ΔE = EF - EB), see Page 3437.

The main inelastic scattering mechanisms are:
         i) Phonon excitation (heat).
         ii) Plasmon excitation (valence electrons).
         iii) Single electron excitation (inner and outer shell scattering).
         iv) Direct radiation losses (Bremsstrahlung radiation due to deceleration of the electron beam in the Coulomb field of an atom).
         v) Excitation of conducting electrons leading to secondary electron emissions.

The ground-state energy of an inner-shell electron is typically some hundreds to thousands of electron volts (eV) below the Fermi level of the solid and the unoccupied electrons states lies only above the Fermi level. When an accelerating electron in the electron beam in EMs interacts with the materials, the inner-shell electrons can be excited and transit to an excited state if the electrons absorb an energy which is equal to or greater than its binding energy. In this case, the accelerating electron in the electron beam loses the same amount of energy and is scattered at an angle. However, these unstable excited electrons will lose its excess energy quickly, producing “byproducts” such as X-rays and Auger electrons. This process is called de-excitation process.

Figure 3960 shows angular distribution of scattered electrons, with inner-shell ionization losses, as a function of scattering angle in EMs.

Inner-shell ionization losses

Figure 3960. Inner-shell ionization losses.

 

Table 3960. Energy losses and scattering angles of various inelastic electron scatterings in electron interaction with materials.
Process Phonon excitation Inter of intra band transitions Plasmon excitation Inner shell ionization
Region of energy loss
Low loss (< 50 eV)
High loss (> 50 eV)
Energy loss E [eV] ~0.02 3 - 25 5 - 25 10 - 2000
Scattering angle θE [mrad] 5 - 15 5 - 10 < 0.1 0.1 - 10
Oscillation Collective oscillations of atoms (e.g. lattice vibrations)   Collective oscillations of free electrons, a quantum of a collective longitudinal
wave in the electron gas of a solid
 
Detectability by EELS Is not resolved Yes Yes Yes
Effects Causes specimen to heat up      
Time     Damped out in < 10−15 s  
Localization     Localized to < 10 nm  
Interaction     Most common inelastic interaction due to high free electron density  
Cross sections     Relatively large Relatively small
Mean-free
paths
    Relatively short Relatively large
Intensity     Much intense Much smaller
Materials     Predominant in metals  
Modification     Can reduce the number of phonons by cooling the specimen.  
Other names Thermal diffuse scattering      
Characteristics Diffuse background, don’t carry any useful information Signature of the structure   Elemental information

 

 

 

 

 

 

 

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