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Alternatively, the objective lens may be overexcited
in the TEM mode (a situation that approximates a STEM
condition), and the probe then focussed with the second
condenser lens. The resulting probe will be smaller than
a conventional TEM probe, but generally, depending on the
amount of overexcitation, larger than the standard STEM
probe (Steeds, 1979). Under this condition, the
diffraction pattern appears in the imaging plane of the
objective lens rather than in the back focal plane. The
lens optics for this condition cannot be found in standard
texts. The reader is referred to Olsen and Goodman (1979)
for details.
The method chosen for this investigation uses the
focussed TEM probe. The beam convergence is controlled by
the size of the second condenser aperture. After
selection and centering of the proper C-2 aperture, the
diffraction mode of the instrument is selected and the
condenser lens is brought to crossover. The result is a
focussed convergent beam and the image will be a focussed
convergent beam diffraction pattern. The beam is not
imagable under this condition. As the beam is focussed
(as the condenser lense is brought to crossover), the
second condenser aperture will become visible as a disc in
the diffraction pattern and a bright field image of the
sample will appear in the central transmitted disc of the
pattern. Dark field images corresponding to the various