Influence of non-dipolar terms on the Cu L(2,3) and M(2,3) electron energy loss fine structure (EELFS) spectra in transmission and reflection mode

Extended energy loss fine structure (EXELFS) spectra, above the M(2,3) and L(2,3) edges, on a Cu polycrystalline thin film, have been acquired by means of a serial spectrometer in a transmission electron microscope. The M(2,3) spectra are compared to the fine structure detected in reflection geometry above the same core edges. In order to single out the influence of multipole terms in the inelastic cross-section in transmission mode, the acceptance angle of the spectrometer has been varied between 2.5 and 100 mrad, corresponding to q(max) ranging from 0.72 to 28.56 Angstrom(-1). The local structural results show no variation in the real space position of the nearest-neighbour atoms, in the backscattering amplitude and phase shift functions. This indicates a strong predominance of the dipolar term in the inelastic cross section, probably due tb the 1/q(4) term which enhances the q(min) contribution. Surprisingly, this holds even for shallow core edges (such as M(2,3)) detected both in transmission and reflection mode, resulting very delocalised (r(a) = 0.2 Angstrom) shell for which the dipolar approximation should not be valid.