Hii^'li Resolution Electron Diffraction Patterns from Microcrystcils 



99 



ELECTRON SOURCE 



APERTURE FOR 

 CONDENSER LENS 1 



nPERTURE FOR 

 CONDENSER LENS 2 



SPEC/MEN 



OBJECTIVE LENS 



OBJECTIVE APERTURE 

 DIFFRftCTION PATTERN 

 FIRST-STAGE IMAGE — 

 OF SPECIMEN 

 SELECTING APERTURE 

 INTERMEDIATE LENS 

 FIRST-STAGE IMAGE OF 

 DIFFRACTION PATTERN 

 SECOND -STAGE IMAGE " 

 OF SPECIMEN. APERTURE 



PROJECTOR LENS 

 SECOND -STAGE IMAGE OF 

 DIFFRACTION PATTERN 



FINAL SCREEN 

 AND PLATE 



BRIGHT-FIELD 

 IMAGE 



O^ERFOCUSED FOCUSED UNDERFOCUSED 



DIFFRACTION PATTERN DIFFRACTION PATTERN DIFFRACTION PATTERN 



ft 



B 



C 



D 



Fig. 1. Ray paths in high resolution micro-diffraction. 



nately, the scattering area cannot be located with 

 sufficient accuracy, as, in order to procure the 

 diffraction pattern, the objective lens current is cut 

 off. Due to this, the stray magnetic fields between the 

 second condenser and the specimen are reduced. 

 These had previously caused a slight deflection of the 

 illuminating beam, and thus a displacement of the 

 image of the electron source relative to the undis- 

 turbed position by a distance large against its own dia- 

 meter. Now, the deflection is equally reduced, which 

 also reduces the displacement of the image of the 

 source, and the identity of the scattering area with 

 the selected field of the electron optical image is 

 lost. This effect may only be overcome by a tedious 

 and elaborate adjustment of the microscope, the 

 success of which is not guaranteed. The arrange- 

 ments of V. Ardenne and co-workers (2) and of Hillier 

 and Baker (4) only permit shadow-microscopic ob- 

 servation of the specimen and moderate diffraction 

 resolution (R ^ 150). 



Experimental arrani^'cnients and teclniic/iie. — We 

 have used an electron microscope type "Elmiskop 1" 

 of Siemens & Halske AG (11). Objective lens and 

 intermediate lens are operated independently. The 

 diameters of the apertures selecting the specimen 



field, which are situated just above the intermediate 

 lens, were chosen as to permit the examination of 

 areas 2.2 and 0.9 /< in diameter. A diagram of ray 

 paths is given in fig. I. The precision of the repro- 

 duction of the camera length (57.5 cm) in routine 

 operation has been discussed (10). 



For high resolution diftYaction work, a small angle 

 of illumination is indispensable. This was realized 

 by using the first condenser lens to obtain a 50 times 

 demagnified image of the electron source (fig. 1, B, 

 C, D). The lens was fed by a 220 V battery, as its 

 feeding by the microscope power supply is not pro- 

 vided when the intermediate lens is operated inde- 

 pendently. This was feasible, because the require- 

 ments regarding current stability are not stringent 

 for demagnitication work. With the second ctinden- 

 ser lens operating at a very long focal length, the 

 angle of illumination is 5 10" rad, giving a dif- 

 fraction resolution of 8350 at 80 kV. To obtain elec- 

 tron optical images on the final screen, which are 

 sufficiently bright for visual observation, the angle 

 of illumination was enlarged by increasing the 

 current of the second condenser lens (fig. 1, A). 



Axial astigmatism of the intermediate lens is 

 detrimental to the diffraction resolution (10). When 



