Abstract
In electron energy loss spectroscopy (EELS), the excitation of crystal field transitions in 3d transition metal oxides takes place via two scattering mechanisms, i.e. direct scattering as well as electron exchange scattering. In addition to spin-polarized EELS, spin-integrated but angular resolved experiments may be helpful to distinguish between exchange scattering and direct scattering using the angular dependence of their scattering amplitudes. Intensity maps of crystal field transitions over the whole 2π solid angle of NiO show characteristic features that can be attributed to the contributions of both scattering amplitudes. It was found that the energy loss at Δ E=2.7 eV, which consists mainly of triplet–singlet transitions and is therefore excited only by electron exchange, shows the angular characteristics that were predicted by theory for the spin flip part of the scattering amplitude. In contrast, the angular dependence of the triplet–triplet transition at Δ E=1.6 eV, which is excited via direct scattering as well as exchange scattering, is similar to the LEED pattern of the zero loss peak, i.e. the dipole-like forward scattering is modulated by reciprocal lattice vectors.
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