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Validity of the dipole approximation in TEM-EELS studies.

R F Egerton1, R A Mcleod, M Malac

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|July 22, 2014
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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.

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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.