| dc.description |
The development of direct-recording electron diffraction systems to permit the exclusion of all but elastically diffracted electrons is described. A retarding field analyser, located at a point in the system beyond the defining aperture, permits simple velocity filtering with no loss of angular resolution or accuracy of intensity measurement, in the profiles of diffracted beams. Debye-Scherrer patterns, from elastically diffracted electrons only, have rings appreciably clearer than the rings in patterns containing elastic and inelastic electrons (that is, the patterns which are obtained in conventional photographic diffraction cameras). The filtered profiles have narrower peaks than the unfiltered profiles and the contrast between peaks and background is enhanced. At low values of sin θ/λ in particular, when the inelastic background is very intense, its removal reveals diffraction rings which were previously invisible.The filter system can be extended, though with increased noise, to give electron diffraction profiles corresponding to groups of electrons which have suffered a given energy loss. For this, an a.c. sorting technique, due to Leder and Simpson, is applied. |
