ABSTRACT
In the previous chapters dichroism has been demonstrated as an effect by which
changing particular excitation conditions (polarization for photons, scattering
angle for electrons) will give rise to differences in spectral features. For both
Electron Energy-Loss Spectroscopy (EELS) [Egerton (1996)] and X-ray Absorption
Spectroscopy (XAS) the absolute circular dichroic signal Δσ is defined as the differ-
ence between two spectra (σ+ and σ−) acquired with opposite helicity; following
this definition Δσ(E) is, like σ+(E) and σ−(E), a function of the energy-loss, but
the energy dependence is usually omitted. In practice it is often more convenient
to refer to the relative dichroic signal σdich as the absolute difference normalized to
the sum of the spectra1:
σdich := σ+ − σ−
σ+ + σ− :=
Δσ
2 〈σ〉 . (9.1)
Since Δσ is generally non-zero only at the energy losses of certain elemental
edges, it is often given as a simple number, referred to as “percentage asymmetry at
the edge”. For example, the dichroism of Fig. 9.5(b) can be given as 7% asymmetry
at the Fe L3 edge. In such cases however, it is always important to indicate whether
peakmaxima or integrals (and their integration interval) are used for this estimate.
