科研之友
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摘要
The electron energy-loss near-edge structures (ELNES) measured using a transmission electron microscope (TEM) generally depend on the orientation and the convergence of the electron beam and on the collector aperture. Within the dipole approximation, the current of inelastic electrons emitted from a particular atomic core level can be expressed as a linear combination of some particular electron inelastic-scattering cross sections (the intrinsic components) calculated for particular orientations of the transferred wave vector. In the case of a very low symmetry around the probed atom (cases of trichroism), the collected current can be expressed as a combination of three, four or six intrinsic components, a situation more complicated than the well-known situation of dichroism where only two intrinsic components exist. This paper aims at investigating situations of very low symmetry. At first, we examine the case of the O K-edge in the rutile form of TiO2, where the local symmetry around the oxygen site is described with the C-2v point group. We are in a situation of (a) trichroism, where the collected current depends on three intrinsic components. These three components are evaluated using either the FEFF8.2 code or the TELNES subroutine of the WIEN2K code. From these components, the collected current can be calculated for any beam orientation and any collector aperture. These simulations are compared with experimental O K near-edge structures recorded in a TEM for different orientations and collection conditions. The simulations of the collected current are in good agreement with the experimental results, and orientation as well as collection angle effects on the ELNES are evidenced. Moreover, it is clearly shown that, in such a case, the relativistic velocity of the incident electrons cannot be neglected. It clearly appears that, neglecting these relativistic effects induces significant changes in the ELNES, leading to a marked deterioration of the agreement between experiment and theory. Such changes are also confirmed in another case of trichroism: the O K-edge in monoclinic zirconia.
- 出版日期2008-6
