Full hemispherical intensity maps of crystal field transitions in NiO(001)by angular resolved electron energy loss spectroscopy

In electron energy loss spectroscopy (EELS), the excitation of crystal fiel d 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 e xperiments may be helpful to distinguish between exchange scattering and di rect scattering using the angular dependence of their scattering amplitudes . Intensity maps of crystal field transitions over the whole 2 pi solid ang le of NiO show characteristic features that can be attributed to the contri butions of both scattering amplitudes. It was found that the energy loss at Delta E=2.7 eV, which consists mainly of triplet-singlet transitions and i s therefore excited only by electron exchange, shows the angular characteri stics that were predicted by theory for the spin flip part of the scatterin g amplitude. In contrast, the angular dependence of the triplet-triplet tra nsition at Delta E=1.6 eV, which is excited via direct scattering as well a s exchange scattering, is similar to the LEED pattern of the zero loss peak , i.e. the dipole-like forward scattering is modulated by reciprocal lattic e vectors. (C) 1999 Elsevier Science B.V. All rights reserved.