Structure and transformation: Large molecular clusters as models of condensed matter.

This paper reviews investigations of homogeneous nucleation in phase transitions in large molecular clusters. The principal techniques brought to bear are electron diffraction analyses of transformations in clusters formed by condensation of vapor in supersonic expansions and computer simulations of spontaneous phase changes in clusters. Results obtained to date are contrasted with those of larger systems and interpreted in terms of nucleation theory. The review also refers to some unresolved aspects of nucleation theory.

A New Method for Analyzing Powder Diffraction Patterns: Confirmation of a Predicted Phase of SF6.

A recent computer simulation reproduced all of the solid-state transitions known for sulfur hexafluoride and predicted the unknown structure of its coldest phase. Subsequent neutron diffraction experiments that were performed to establish the structure of this phase could not be interpreted by conventional procedures. A method for analyzing Debye-Scherrer diffraction patterns was designed to avoid the many false minima that are encountered in standard analyses of low-symmetry phases. The structure found with this method confirmed the previous theoretical prediction.

Atomic images by electron-wave holography.

Direct views of electron clouds in atoms h-ave been observed visually and photographed by means of a two-stage holographic miscroscope based on Gabor's principle of image reconstruction. Holograms are produced with 40-kilo-volt electron radiation and decoded with an optical laser. The obviation of an objective lens makes feasible a numerical aperture of 0.37 and a resolving power exceeding 0.1 angstrom.