Electron Imaging

Particle/wave dualism is evident in experiments with electrons. They form a beam of particles, for instance, in a synchrotron, but behave as waves in an electron microscope. Electrons are scattered by matter just as X-rays are. Electrons are scattered by the atomic coulombic potential distribution in the atoms. This scattering is much stronger than for X-rays and can be recorded from very tiny specimens. This is particularly valuable for obtaining information on individual regions extending over only a few unit cells of a two-dimensional crystal. The imaging experiments are performed in an electron microscope at a wavelength between 0.02 and 0.04 Å, much shorter than for X-rays. Then, the Ewald sphere  can be regarded as a flat surface, and diffraction from a stationary crystal shows essentially the reflections in a planar section of reciprocal space. By tilting the specimen, several intersections can be obtained and combined in a more complete image of reciprocal space. The short wavelength would result in a very high image resolution if the quality of the microscope lenses were not a limiting factor. At best they allow reaching a resolution of 1.5 Å. Simple switching of lens currents changes the observation from a diffraction pattern to a real image.

Transfer of energy to the specimen and the resulting radiation damage is a limitation in applying electron scattering. This is especially serious in studying biological material. However, with very thin specimens of two-dimensional crystals and with the molecules embedded in vitreous ice or glucose, extremely interesting results have been obtained from biological specimens by short exposure times, low beam intensity, cryocooling, and a combination of diffraction data and image analysis. The phases of the structure factors of the diffracted beams are calculated from the image. They are combined with the measured amplitudes of the diffracted beams and, exactly as in X-ray diffraction, a Fourier summation gives the atomic distribution in the specimen. At the present time, the maximum resolution is 3 Å.

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