Electron energy loss spectrometry (EELS) allows one to experimentally obtain the dielectric permittivity 𝜀:(ω, q) as a function of frequency ω and wave vector if. From 𝜀 information on inter band transitions in the probed medium can be drawn. In EELS, inter band transitions are screened by the movement of the loosely bound valence or conduction electrons. The screening effect may enhance or attenuate the strength of transitions, and tends to shift the frequencies of resonant oscillations. Another aspect of screening is the occurrence of longitudinal modes in the spectrum. So, great care has to be taken in interpreting loss spectra. Examples are discussed for image mode spectra and for diffraction mode spectra which latter render investigation of non-vertical transitions, and hence tests of band structure calculations possible.
Schattschneider, P. and Pongratz, P.
"Interband Transitions in Electron Energy Loss Spectrometry,"
Scanning Microscopy: Vol. 1990
, Article 3.
Available at: https://digitalcommons.usu.edu/microscopy/vol1990/iss4/3