Electron Phase Contrast Images Of Molecular Detail

Electron phase contrast images with a resolution of at least 2 A have been obtained using a modified commercial electron microscope. A uphase column" approximation for interpreting such images is briefly discussed and applied to images of graphite, evaporated carbon, and a synthetic polypeptide. Hexagonal features about 5 A in diameter are attributed to the graphite unit cell imaged by the six first-order prism plane reflections. The image so produced is a next-nearest neighbor representation. The steady improvement in resolving power of commercial electron microscopes over the past few years has re-awakened considerable interest in the possibilities of directly imaging details of molecular structure. In particular, the phase contrast mechanism based on the Abb6 theory of image formation expressed in terms of the Kirchoff diffraction integral has been re-examined using numerical computation methods not in wide use when Scherzer 1 discussed phase image formation. Several theoretical papers 2 3 * 8 dealing with phase contrast images of atom positions have indicated that it should be possible to experimentally obtain such images under the right conditions. The computations all assume a monolayer specimen in the object plane or effectively a single atom approach. This idealized specimen is difficult to realize experimentally and for that reason this brief account is concerned with problems and some results with actual three-dimensional preparations. High resolution Fourier or "lattice" images of near perfect three-dimensional crystals 6,7 about one-fourth extinction distance thick have become familiar o in the last two to three years with image detail exhibited down to 1.8 A.