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Validity of the dipole approximation in TEM-EELS studies
R F Egerton1, R A Mcleod, M Malac
1Physics Department, University of Alberta, Edmonton, Canada, T6G 2E1.
Microscopy Research and Technique
|July 22, 2014
Summary
Nondipole effects in electron energy-loss spectroscopy (EELS) are minor deviations from Lorentzian distributions. These effects are often masked by plural scattering but can influence measurements, especially off-axis.
Area of Science:
- Physics
- Materials Science
- Spectroscopy
Background:
- Established electron energy-loss spectroscopy (EELS) procedures assume Lorentzian angular distributions for inelastic scattering.
- Deviations from this assumption, termed nondipole effects, can impact data analysis.
- Plural scattering (combined elastic and inelastic) is a significant factor in EELS data, often not fully deconvoluted.
Purpose of the Study:
- To evaluate nondipole effects in EELS by analyzing deviations from Lorentzian angular distributions.
- To assess the impact of these deviations on established EELS analysis techniques.
- To understand the conditions under which nondipole effects become significant in EELS.
Main Methods:
- Analysis of electron scattering angular distributions in EELS.
- Evaluation of deviations from Lorentzian scattering models.
- Consideration of generalized oscillator strength and Bethe-ridge distributions.
- Assessment of retardation effects at high incident energies (>200 keV).
Main Results:
- Nondipole effects manifest as small deviations from Lorentzian angular distributions in EELS.
- These deviations may be less significant than the influence of unremoved plural scattering.
- Non-Lorentzian behavior in the core-loss region arises from reduced generalized oscillator strength and Bethe-ridge formation.
- Retardation effects at high energies further distort angular dependence.
- Nondipole effects are typically masked by intensity falloff with angle for on-axis measurements but can be important for off-axis collection.
- Near-edge fine structure in EELS is sensitive to nondipole effects.
Conclusions:
- Nondipole effects in EELS are generally small but can influence specific measurements, particularly off-axis.
- Conventional deconvolution methods may not fully account for plural scattering, potentially overshadowing nondipole effects.
- Minimizing nondipole effects in near-edge fine structure analysis can be achieved using angle-limiting apertures.
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