Some of the interesting features inherent to precession:
1。The pattern may be indexed as a conventional diffraction pattern while the intensities have actually been gathered from off-zone reflection conditions.
2。Non-systematic dynamical effects such as Kikuchi lines and intensity variations in CBED spots are reduced by averaging over incident beam directions.
3。Since the beam is entering the sample from an off-axis direction, much of the dynamical scattering that is particularly strong at the exact Bragg condition (or zone axis channeling condition) is avoided. Typically only one strong beam is simultaneously illuminated with the transmitted beam (two-beam condition). In some cases three beams may be strongly illuminated, but no clear systematic dynamical path exists so the interaction between beams is not strong.
4。Many more FOLZ reflections are illuminated, under more kinematical conditions, by the Ewald sphere allowing the acquisition of an increased number of intensities for use in structure solution techniques.
5。HOLZ reflections are illuminated, yielding expanded 3-dimensional data sets provided that spots from separate Laue zones do not overlap.
We have built two precession systems for conducting PED experiments. One was retrofitted on a Hitachi H-9000 UHV microscope and the other was installed on a JEOL 2000FX and later moved to a JEOL 3000F. Some experimental images are shown below demonstrating the effects of the precession technique. They are diffraction patterns from a very thick Mg3V2O8 crystal. They have been taken under identical environmental conditions, processed in the same bath, scanned under the same conditions, and processed identically in Photoshop. A very moderate precession angle (~ 5 mrad) was used to form the right-hand pattern to demonstrate the effects of precession. The extension by precession of the first order Laue ring into an annulus of width 10 mrad is clearly seen, as well as the blending of non-systematic dynamical effects into a radially diffuse background. This dynamical background averaging can improve intensity measurements by considerably simplifying the problem of background subtraction. One can also see that the reflections in the ZOLZ are more clearly defined. It is also apparent that dynamical scattering still contributes to the image, so thin specimens are preferred, however correction factors can be applied if the specimen is thick if some of the structure factors are known.