Calculation of electron-energy-loss spectra for 3s→3d excitation in transition metal oxides

We present theoretical calculations of electron-energy-loss spectra (EELS) for 3s --> 3d excitation in 3d transition metal (TM) oxides Cr2O3, Mn2O3, MnO, COO, and NiO. Both direct and exchange scattering processes are taken into account. Multiplet couplings with the cubic crystal field is considered to describe the TM systems. The spectral structure is determined by the 3s3d(n+1) multiplet. The 3s --> 3d excitation is a quadrupole transition, which is hardly observable by photo-absorption. The calculated results are compared with the experiments by Steiner et al., in which an exchange-split double-peaked structure is observed for less-than-half d count systems, Cr2O3 and Mn2O3, and a single-peaked one for more-than-half, MnO, COO, and NiO. It is found that the direct scattering process, which conserves the spin direction, dominates the excitation process, so that the intensities of the double peaks are roughly proportional to the d hole numbers with up and down spins. The exchange scattering is shown to increase in low incident electron energy, although it gives a weak contribution even at the threshold excitation in the present systems. (C) 1998 Elsevier Science B.V.