DOUBLE CONICAL BEAM-ROCKING SYSTEM FOR MEASUREMENT OF INTEGRATED ELECTRON-DIFFRACTION INTENSITIES

The performance of a scanning system for a transmission electron microscope is described, where the focused beam is scanned at a constant angle around the optic axis. Below the specimen, the beam is de-scanned, the net effect being equivalent to precessing the specimen around a stationary beam focused to a probe size of 0.1 mu m for typical precession and convergence angles. Unlike standard convergent beam electron diffraction (CBED) patterns aligned on a zone axis, precessed patterns include many reflections intercepted by the Ewald sphere, where the diffracted intensities are determined by integration through the Bragg condition. The double conical scanning system was designed to collect a large data set of integrated intensities that are more suitable for structure determination by electron diffraction, both by removing excitation errors due to curvature of the Ewald sphere and also by reducing non-systematic dynamical effects. Experimental patterns obtained from the Si{110} axis are discussed, followed by analysis of the intensities diffracted into the higher order Laue zones of a rare earth pyrogermanate (Er2Ge2O7) with a large unit cell. The scanning system is equally suitable for collection of precessed spot patterns from radiation-sensitive specimens such as organic crystals.