ELECTRON-SCATTERING EXPERIMENTS USING A POSTCOLUMN IMAGING ELECTRON-ENERGY FILTER

An imaging electron energy filter in combination with a conventional e lectron microscope is used to record the intensity distribution of ela stically scattered electrons. To automate the intensity acquisition to a high degree and to allow pixel-precise operations, the whole system is operated under remote control by external computers using a script -based program. The intensity is recorded on a slow-scan CCD camera wi th high linearity, and integration of pixel intensities results in a h igh S/N ratio. Subsequent acquisition of parts of the diffraction patt ern yields a high dynamic range (19 bits), and large scattering length s of 300 nm(-1) can be achieved. The intensities are used to calculate the pair distribution function (PDF) with an accuracy of 0.03 nm in r eal space. Atomic pair distances up to I nm can be determined. The met hod is tested at polycrystalline gold and nickel films, and it is appl ied to vitreous silica and amorphous carbon. The results are in very g ood agreement, for example to the PDF obtained from neutron scattering at vitreous silica. (C) 1998 Elsevier Science B.V. All rights reserve d.