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Validity of the dipole approximation in TEM‐EELS studies
Author(s) -
Egerton R.F.,
Mcleod R.A.,
Malac M.
Publication year - 2014
Publication title -
microscopy research and technique
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.536
H-Index - 118
eISSN - 1097-0029
pISSN - 1059-910X
DOI - 10.1002/jemt.22398
Subject(s) - dipole , discrete dipole approximation , materials science , nanotechnology , physics , quantum mechanics
Nondipole effects in electron energy‐loss spectroscopy are evaluated in terms of deviation of the inelastic scattering from a Lorentzian angular distribution, which is assumed in established procedures for plural‐scattering deconvolution, thickness measurement, and Kramers‐Kronig analysis. The deviation appears to be small and may be outweighed by the effect of plural (elastic + inelastic) scattering, which is not removed by conventional deconvolution methods. In the core‐loss region of the spectrum, non‐Lorentzian behaviour stems from a reduction of the generalized oscillator strength from its optical value and (for energies far above an ionization threshold) formation of a Bethe‐ridge angular distribution. At incident energies above 200 keV, retardation effects further distort the angular dependence, even for core losses just above threshold. With an on‐axis collection aperture, non‐dipole effects are masked by the rapid falloff of intensity with scattering angle, but they may become important for off‐axis measurements. Near‐edge fine structure is sensitive to nondipole effects but these can be minimized by use of an angle‐limiting collection aperture. Microsc. Res. Tech. 77:773–778, 2014 . © 2014 Wiley Periodicals, Inc.