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The EEL spectrum can be described in a dielectric formulation [1] by,
 [3372a]
where,
v  The speed of the incident electron;
n_{a}  The number
of atoms per unit volume;
θ_{E}  The characteristic scattering angle (θ_{E} = E/γm_{0}v^{2});
Im(−1/ε)  The energy loss function
Based on Equation 3372a and a Kramers–Kronig analysis, the complex dielectric function ε = ε_{1} + iε_{2} can be obtained from the lowloss EEL spectrum. Furthermore, with approximation, it can be obtained,
 [3372b]
The TEM specimen thickness can be extracted using KramersKronig sum rule, [2, 3]
 [3372c] where,
a_{0}  The Bohr radius,
E_{0}  The electron energy,
F  The
relativistic factor,
n  The optical refractive index,
β  The
collection angle,
θ ≈ E/(2FE_{0}).
Experimental tests revealed that the thickness calculation by KramersKronig sum rule is accurate to ~10%. [2,3]
The optical absorption α can be given by,
 [3372d]
As discussed on page4360, the energyloss process of incident electrons can be treated by the dielectric response of the electrons. Therefore, the EELS contains information about the dielectric constant ε. The singlescattering spectrum intensity I is given by,
I = I_{0}(t/k)Im(1/ε)ln[1 + (β/θ_{E})^{2}]  [3372e]
where,
I_{0}  The intensity in the zeroloss peak.
t  The specimen thickness.
k – A constant incorporating the electron momentum and Bohr radius.
Figure 3372a shows the EEL spectra of plasmon region (zeropeak is not shown) for crystalline and amorphous diamond. The loss function (Im[l/ε]) can then be obtained by removing contribution from multiple scattering using the Fourierlog deconvolution method.
Figure 3372a. EEL spectra of plasmon region for (a) crystalline and (b) amorphous diamond.
An optical absorption method (see Equation 3372d) can directly provide the imaginary part of the dielectric function, ε_{2}, associated with a single electron excitation of an interband transition, while EELS cannot directly give ε_{2}. Based on the obtained loss function, the real part (ε_{1}), and the imaginary part (ε_{2}) of the dielectric function for the crystalline and the amorphous diamond are extracted using KramersKronig analysis as shown in Figure 3372b. Table 3372 lists the onset energy (indicating the band gap energy) and the peaks in the imaginary spectrum obtained from Figure 3372b and the original interband transitions. In many cases, the broad maxima in ε_{2}, e.g. with energies higher than 10 eV, represent widely distributed oscillator strengths. That is, the peaks in ε_{2} correspond to some interband transitions.
Figure 3372b. Real part (ε_{1}), and the imaginary part (ε_{2}) of the dielectric function for (a) crystalline and (b) amorphous diamond.
Table 3372. Onset energies (band gap energies), peaks in the imaginary spectrum in Figure 3372b and the original interband transitions.

Onset energy (band gap energy) 
Peaks in imaginary spectrum 
Interband transition 
Crystal diamond 
5.5 eV 
8.2 
Γ point 
12.7 
X and L points 
Amorphous diamond 
4.0 eV 
7.2 eV 
Γ point 
[1] R. F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope, Plenum Press, New York, (1996).
[2] Y. Y. Yang and R. F. Egerton, Micron 26, 1 (1995).
[3] R. F. Egerton and S. C. Cheng, Ultramicroscopy 21, 231 (1987).
