Possibilities of heavy atom discrimination using single-sideband techniques

Separation of the images of heavy and light components of a specimen is in principle possible on the basis of the anomalous scattering effect, which is in fact a natural part of the description of the interaction of the specimen and illuminating electrons. Single-sideband techniques provide one possible realization of the discrimination scheme, which can be viewed as the essential part of the solution of the phase problem. The image contrast generated by the anomalous effect is examined using published data for complex atomic scattering factors. The expected contrast is small, and may easily be obscured by noise from various sources. The most severe source of noise is seen to be the structural change of the specimen during exposure of a series of micrographs. Information on specimen stability, then, can be used to estimate chances for success. Discrimination using single-sideband techniques might be expected to work on some specimens of current biological interest, but would be at best marginal at the single atom level. The single-sideband method has been experimentally implemented and tested with a range of specimens, confirming the theoretical predictions. Possible improvements are discussed. (Auth.)