Sampling the diffraction pattern of a ®nite specimen more ®nely than the Nyquist frequency (the inverse of the size of the diffracting specimen) corresponds to surrounding the electron density of the specimen with a no-density region. When the no-density region is bigger than the electron-density region, suf®cient information is recorded so that the phase information can be retrieved from the oversampled diffraction pattern, at least in principle. By employing an iterative algorithm, the phase information from the oversampled diffraction pattern of a micrometre-sized test specimen has been successfully retrieved. This method is believed to be able to open a door for high-resolution three-dimensional structure determination of complex and non-crystalline biological specimens, i.e. whole cells and sub-micrometre molecular clusters and micrometre-sized protein crystals. With the possible appearance in the future of X-ray free-electron lasers, it may become possible to image single molecules by recording diffraction patterns before radiation damage manifests itself.
Abstract-2 -It has long been known that x-ray holography offered the possibility of three-dimensional microscopy with resolution higher than the light microscope and with special forms of contrast based on x-ray edges. In principle, the
SLAC-PJB-5948 DE93 004091 producing a radiation beam witha brightness eight orders ofmagnitude larger than synchrotron light sources, and pulse duration shorter than one picosecond. Such a large increase in radiation A 2 to 4 nm HIGH POWER FEL ON THE SLAC LINAC* flux, and the short pulse duration, will open new exciting and unique research possibilities in C. Pellegrini, J. Rosenzweig physics, biology and other sciences.
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