Influence of non-dipole transitions on the extraction of EMCD

Abstract

ELNES-spectra of the L3 and L2-edge of bcc-iron are simulated using a Bloch wave method for the calculation of the DDSCS within a 1st-order Born approximation of the inelastic electron scattering process in order to study the influence of non-dipole terms on the EMCD-signal. The necessary electronic structure information is obtained from a DFT-calculation using WIEN2k.Two different different ways of extracting the EMCD-signal are considered: the pEMCD-signal, which is extracted from the pure imaginary part of the MDFF, and the eEMCD-signal, extracted via an EMCD difference method.A non-negligible contribution of the 13 cross term to the eEMCD-signal is found. Furthermore it is shown that the double difference method and the single left-right difference method cancel out the contributions of the 01-term to the eEMCD-signal. The pEMCD-signal is found to be influenced by non-dipole terms only for large scattering angles. Conclusive quantitative results on the influence of non-dipole contributions to the eEMCD-signal remain to be found, however, since it is uncovered that the eEMCD-signal is strongly disturbed by the choice of the post-edge normalization range due to the inaccurate description of the post-edge region in the present simulation. Furthermore a not anticipated "apparent anisotropy" of the real part of the MDFF is found, whose cause is presently unknown.As a byproduct of these investigations deeper insight is gained on a reason, why the double difference method is superior to the other extraction methods. It practically eliminates the effect of non-dipole terms 01 and 12.Lastly two effects are encountered that might pave the way to a deeper understanding of why the L2-edge is experimentally often observed to be weakened or suppressed relative to the L3-edge in comparison with simulations

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Last time updated on 04/03/2019

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