THE EFFECT OF PENDELLÖSUNG OSCILLATIONS ON THREE-BEAM DIFFRACTION PROFILES. K. Hümmer and E. Weckert, Institut für Kristallographie, Universitåt Karlsruhe (TH), Kaiserstr. 12, D-76128 Karlsruhe, Germany

Three-beam diffraction provides a means of physical determination of triplet phases by interference of simultaneously excited waves inside a crystal. Recently triplet-phase determination of small protein crystals became feasible. Usually crystals of arbitrary shape with grown faces are used. Therefore, the diffracted beams will be simultaneously in Bragg as well as in Laue diffraction geometry. In the Bragg reflection case the interference profiles do not significantly depend on the thickness of the crystal. Theoretical calculations using plane wave dynamical theory taking into account the divergence and wavelength spread of the incident beam and experiments done at a high brilliance synchrotron source (ESRF, France) correlate nearly quantitatively. In the Laue transmission case, however, theoretical plane-wave calculations predict that interference effects will be affected by Pendellösung oscillations if crystal thickness exceeds the first maximum of the Pendellösung. Experiments performed with 'perfect' plate-shaped crystals of benzil at different wavelength confirm this theoretical predictions. However, the dependence of the interference effects on the thickness of the crystal is weaker than predicted theoretically. Two reasons may be considered for this behaviour: a. The divergence of about 2 arcsec used for the experiment is large compared to the extremely narrow dynamical reflection width of organic crystals and therefore the assumption of plane waves is a rough approximation. b. Small imperfection do already hinder the formation of Pendellösung interferences. Therefore, one can be sure, that in practice three-beam diffraction can be unambiguously exploited for triplet phase determination even of macromolecular structures.