Core-Shell Excitation EELS of High-Angle Phonon-Scattered Electrons

Abstract

A new technique, thermal-diffuse-scattered electron energy-loss spectroscopy (TDS-EELS), has been demonstrated to be useful for measurement of atomic mean vibration amplitude aA. This technique is performed in the dark-field image mode with a large-angle, annular objective aperture in a conventional transmission electron microscope (TEM) (Figs. 1 and 2). TDS-EELS is based on high-angle phonon - atomic inner-shell double inelastic (rather than high-angle atomic inner-shell single inelastic) scattering processes, and has shown significant temperature dependence (Fig. 3), which may arise from the variation of aA with temperature.Signals in TDS-EELS could be generated by the following scattering mechanisms. One is the “coherent” double-inelastic scattering process, in which TDS and core-shell excitations occur “simultaneously” during scattering by the same atom. The other is “incoherent” double-inelastic scattering, in which TDS and core-shell excitations occur separately during scattering by different atoms. If the incoherent double scattering is the dominant process in TDS-EELS and if the collection angular range of the annular objective aperture is large (>30 mrad), the ratio of the integrated ionization edge intensities would be expected to be independent of temperature and similar to the intensity ratio of the corresponding ionization edges acquired in bright-field (BF-EELS) mode, where I(A)/I(B) ≈ nA/nB·σ(A)/σ(B), σ is the integrated ionization cross-section and n is the atomic concentration.