Interpretation of electron diffraction data from organic microcrystals: a theoretical perspective.
The current framework for the quantitative analysis of transmission electron diffraction data from organic microcrystals is discussed. The appropriate formulation depends upon the crystal perfection: solution-grown material tends to be nearly perfect with regular elastic bends whilst the epitaxial samples are usually considerably less perfect with highly irregular bends. It is possible to obtain Bragg intensity data which are near-kinematical and thus amenable to ab initio structure analysis. Thereafter, the further refinement of the model proceeds with a consideration of dynamical and crystal bending effects. If both of these are small it is a good approximation to analyse them independently rather than use the correct description of dynamical scattering from a deformed foil. The analysis of three-dimensional data sets from tilt series is considered. Although the technique has been successfully demonstrated in several instances there are still details of the analysis that need further clarification.
Direct phasing of electron diffraction data from organic crystals: the effect of n‐beam dynamical scattering
The conditions under which direct phasing methods might be used to determine the crystallographic phases from electron diffraction intensity data are investigated through numerical calculations for two organic crystals. The Cowley-Moodie multislice formulation of dynamical diffraction theory is used to calculate the diffraction intensity data for various crystal thicknesses, at electron energies of 100 keV and 1.0 MeV. The direct phasing method is found to give generally correct phases up to a crystal thickness of about 75 A at 100 keV. The use of higher electron energy (i.e. 1.0 MeV) produces a significant improvement in the success of the direct phasing approach at crystal thicknesses greater than 75 A.
Noctilucent cloud observations over Greenland by a Rayleigh lidar
We report on the detection of noctilucent clouds (NLCs) by a Rayleigh lidar system located on the west coast of Greenland during operations in July and August 1994. The lidar detected NLCs in the zenith on three nights (July 28–29, July 31–August 1, and August 4). The lidar measurements are presented at temporal (1 minute) and spatial (192 meter) resolutions exceeding previously reported lidar measurements of NLCs and reveal significantly more structure in the clouds. Unique cloud features are observed for each night NLCs were detected. Because of the large backscatter ratio associated with NLCs, the volume backscatter coefficient for the cloud particles can be determined more accurately from the lidar signal than the backscatter ratio as the upper mesospheric molecular number density is negligible in the volume backscatter coefficient calculation. The volume backscatter coefficient values determined from the three nights of NLC observations would be classified by current models as moderate to extreme. The extreme backscatter from NLC measurements on August 4 tests the assumptions used in existing models.